Many of the unicellular organisms have a greater concentration of osmotically active material within their body fluids than there is in the medium surrounding them, and the higher animals have glands like the kidney and sweat glands which are able to elaborate fluids with much lower concentrations of sodium and chloride than the body-fluids from which they were derived. The concentrations of sodium and chloride in hypotonic urine and sweat are much lower than those in the serum, and consequently the total osmolar concentrations are also lower, but the urea is characteristically higher. There is no proof that the process is one involving the secretion of water; and, indeed, the kidney is now thought to produce a hypotonic urine by the active reabsorption of sodium from the distal tubule, the walls of which are impermeable to water unless there is posterior pituitary hormone in the circulation.

The foetal fluids and membranes deserve more consideration in this connexion than they have hitherto received. Jacqué (1903–4, 1905–6) pointed out that the allantoic fluid of the sheep was hypotonic and propounded the theory that this fluid was largely foetal urine which he also demonstrated to be hypotonic. This idea has been brought up to date by McDougall (1949) and by Alexander, Nixon, Widdas, & Wohlzogen (1955 a, b). Paton, Watson, & Kerr (1908–9) discussed the whole question of the origin of foetal fluids—human and otherwise—in the light of the knowledge which existed at that time. There is no doubt, as they pointed out, that these fluids are of foetal origin, but the progress of knowledge as to their nature and origin has always been obscured by (i) the great variability in their volume and composition even between littermates and certainly with the progress of gestation, (ii) obvious species differences, (iii) a tendency to compare their composition with foetal urine, rather than with foetal or maternal serum, (iv) a failure to appreciate the importance of their total osmolar concentration, and (v) neglecting to search for general principles and generalizations.

Davies & Routh (1957) have recently made out a strong case for regarding the allantoic fluid of the rabbit as the product of the foetal mesonephros. Their essential findings have been confirmed (Dickerson & McCance, 1957) and the fluid shown to be slightly hypotonic to the maternal serum. Wislocki (1935) made an elaborate study of the volume of allantoic fluid and of amniotic fluid in the pig and cat. The volume relationships in the two species were not the same. Wislocki drew attention to the fact that the allantoic fluid of a pig foetus might exceed 200 ml. before the embryo was 3 cm. long and added that ‘this circumstance in the sow cannot be reconciled with the theory of the allantoic fluid being in whole or major part an excretory product of the mesonephros’.

McCance & Widdowson (1953) pointed out that there were grounds for believing that the foetal kidney of the sheep and man, and probably of other species, constantly produced a very hypotonic urine in relatively large amounts and that this must involve the secretion of water by the placenta into the foetus or the removal of sodium and chloride from it. They suggested (McCance & Widdowson, 1954) that the pig’s allantoic membrane carried out the same function while the placenta was still undeveloped, and that this accounted for the low osmolar concentration found by them in the pig’s allantoic sac about the 24th day of pregnancy. Some further work has now been done on the composition of the allantoic and amniotic fluids of the pig.

Eleven cross-bred Large White x Wessex or Essex sows were used to provide material for these studies. They were killed at the slaughterhouse 20–65 days after service. The uterus was removed as soon as possible after death and in most cases a sample of maternal blood was also collected. The uterus and blood were taken without delay to the laboratory where the serum was separated and the uterus dissected. Each foetus was transferred to a porcelain dish and samples of allantoic fluid, and in later stages also of amniotic fluid, were withdrawn with a syringe for analysis. The volume of fluid withdrawn was noted and the volume still remaining in the sac was then determined by rupturing the sac and pouring the fluid into a measuring cylinder. Samples of fluid for analysis were preserved under toluene.

The physical and chemical methods used in the analysis of the fluids were those given by Dickerson & McCance (1957) with the additional estimation of creatinine and phosphorus by modifications of the methods described by Hawk, Oser, & Summerson (1947).

1. The effect of age

Table 1 shows the composition of the allantoic fluid at several stages of gestation. In many ways the results are incomplete but the experiments were ranging ones and the important and suggestive findings will be elaborated later. The points to note are: (1) at 20 days the volume was small, the total osmolar concentration 175–290 m.osmols/1., with an average value of 256, and the concentration of urea in the fluid nearly always lower than that in the maternal body-fluids; the concentration of creatinine was always lower. The uterine fluids, which may perhaps be regarded as the medium surrounding the sow’s foetus at this age, were analysed on two occasions. The concentrations of creatinine in them were found on both occasions to be the same as those in the allantoic fluids and the concentrations of urea to be the same on one occasion and slightly higher on the other. The figures obtained were: for creatinine on the first occasion T3 and 1*4 mg./100 ml. for the two fluids and on the second occasion 1 ·5 and 1·5 mg./100 ml. respectively; and for urea, on the first occasion 2·6 and 2·7 m.mol. /I. and on the other 3·45 and 3·1 m.mol./I. respectively. As in serum the concentration of sodium ions in the allantoic fluids exceeded that of chloride ions. (2) At 41–46 days the volumes were larger, as Wislocki had shown, and the total osmolar concentrations considerably lower, for at this age all were below 150 m.osmols/1. whereas at 20 days they were all above. The concentration of urea in the fluids was higher than in the sow’s body-water and the concentration of creatinine chromogens considerably higher. The fall in the total osmolar concentration was largely due to the fall in the concentration of sodium ions. The potassium did not fall to the same extent and neither did the chloride, and at this age the chloride ions frequently exceeded the sum of the sodium and potassium ions. The essential findings shown at this age are all confirmations of results obtained on three other litters which had been analysed previously but less completely. The limited observations made on foetuses aged 55–65 days take the changes a stage further than those noticeable at 46 days. Although sows’ serum was not analysed at this age, it may be assumed that its composition remains approximately constant throughout pregnancy. The average volume of allantoic fluid was considerably higher than at 41–46 days and the osmolar concentration lower. The percentage of sodium and chloride in the fluids had fallen farther and that of urea had risen a little. The concentration of chloride ions did not always exceed that of sodium and potassium at this age but it was always greater than that of sodium alone.

TABLE 1

The effect of age on the composition of the allantoic fluid

The effect of age on the composition of the allantoic fluid
The effect of age on the composition of the allantoic fluid

2. The effects of volume

Table 2 shows the composition of the largest and smallest volumes of allantoic fluid in three litters. It is to be noted here that: (1) there were great variations in volume; (2) this did not materially alter the concentration of urea within the fluids; but (3) it made a considerable difference to the concentrations of electrolytes, and although the osmolar concentrations were not measured, values for the minerals show that the larger volumes had lower ones. Age, however, was probably more important in defining composition than volume.

TABLE 2

The effect of volume on the composition of the allantoic fluid

The effect of volume on the composition of the allantoic fluid
The effect of volume on the composition of the allantoic fluid

3. The amniotic fluid

Table 3 shows results which have been obtained on the amniotic fluid. At the two later age periods its composition relative to allantoic fluid may be seen by referring to Table 1. The more gradual increase in the volume of this fluid, as compared with that of allantoic fluid, is again in agreement with the findings of Wislocki (1935). Its volume at 46 days averaged only 26 ml. The total osmolar concentration was always over 260 m.osmols/1. In man, Makepeace, Fremont-Smith, Dailey, & Carroll (1931) suggested that the amniotic fluid may be isotonic with maternal serum early in gestation and be diluted later with foetal urine. In the pig we have found no evidence of isotonicity with maternal serum but the possibility of the amniotic fluid being isotonic with foetal serum at some stage has not been excluded; indeed, the one value obtained for the total osmolar con centration of foetal serum at 46 days was the same as the average figure for the total osmolar concentration in the amniotic fluids at this age, but there was a considerable scatter in the osmolar concentrations among the individual fluids. On the two occasions when maternal serum and amniotic fluid were analysed side by side, the concentrations of urea, creatinine, and phosphorus were lower in the amniotic fluid than in the maternal body-fluids. The concentration of sodium and chloride ions approximated to those found in the maternal body water. The concentration of potassium ions was higher in the amniotic fluid than in maternal serum and at 46 days had the same average value as the cord plasma, which was considerably higher than the figure for the maternal serum (Widdowson & McCance, 1956). The fluids obtained in the period 55–65 days had a lower concentration of potassium ions than those at 46 days.

TABLE 3

The effect of age on the composition of the amniotic fluid

The effect of age on the composition of the amniotic fluid
The effect of age on the composition of the amniotic fluid

The variability in the composition and volume of the pig’s allantoic fluids with age is at first confusing, but the work carried out on them up to date makes it legitimate to put forward the following statements about them. (1) Their composition relative to that of the maternal or foetal sera shows that they are certainly formed by a process of secretion rather than dialysis. (2) The allantoic fluids have initially a small volume and an osmolar concentration not far removed from that of the serum. Between the 20th and 41st day they increase greatly in volume and decrease considerably in osmolar concentration. The latter change is due mainly to a fall in the concentration of sodium ions and to a lesser extent of chloride ions, but there is also a fall in the concentration of potassium. There must be some mechanism in their formative cells for secreting free water into the foetal fluids or for removing sodium, potassium, and chloride from them. The latter seems more probable. The effects of these mechanisms become progressively more conspicuous between the 46th and 65th day of gestation. Attempts were made to demonstrate the secretion of water and (or) the reabsorption of sodium by the allantoic membranes in vitro, but these were not successful in a saline phosphate medium (Robinson, 1949) whether glucose and glutamic acid had been added or not. (3) The fluids are often of a size relative to the foetus which makes it very difficult, if not impossible, to regard them as products of the kidney (Wislocki, 1935; McCance & Widdowson, 1954). They must in all probability be formed by the allantoic membranes themselves, although their composition may be modified by the activities of the foetus. The physiological inter relationship between these formative cells of the allantoic fluids and the water metabolism of the foetus has still to be explored. Great species differences must be expected. (4) The concentration of urea at 20 days is usually below that of the maternal serum but not that of the uterine secretions. By the 46th day the con centration in the fluids is higher than that found in the maternal or foetal body water. As in the formation of sweat, therefore, water must be removed at some stage or urea added. The latter is perhaps more probable but there is no evidence one way or the other. These statements are true also of the creatinine chromogens. If the changes in volume are taken into account it is clear that the fluids contain much more urea and creatinine chromogens at the latter than at the former date, and since the concentration of urea in the fluids has by 41–46 days risen above that in the foetal or maternal serum, the escape of urea from the fluids must be less rapid than it is across the placental barrier. The source of this additional urea and creatinine chromogen may be the foetal urine but this remains to be proved.

Judging by their volume and composition the origins of the amniotic fluids must differ from those of the allantoic fluids. In early pregnancy the amniotic fluids increase relatively slowly in volume and change little in their total osmolar concentration. The fact that at this age the concentrations of urea and creatinine chromogens in the fluids are below those in the maternal or foetal serum speaks against their having a renal origin. It also suggests that some secretory mechanism must be involved although the concentrations of electrolytes are of the order to be expected in an ultrafiltrate or dialysate.

The foetal fluids of the pig have been investigated at 20,46, and 65 days after mating.

  1. The allantoic fluid has been found:

    • to increase rapidly in volume and decrease in tonicity between 20 and 65 days. The decrease in tonicity was largely due to the fall in the concentration of sodium ions;

    • to contain lower concentrations of urea and creatinine at 20 days than the parent body-fluids, but to contain higher concentrations at 46 days;

    • at 20 days to contain, like serum, an excess of sodium ions over chloride ions but at and after 46 days always to contain an excess of chloride ions over sodium ions and sometimes over sodium + potassium;

    • to differ greatly in volume from one foetus to another even within the same uterus. This difference in volume was often accompanied by considerable differences in the concentrations of electrolyte but not in that off urea.

    These findings suggest that the allantoic fluid may be produced initially by a secretion of the allantoic membranes and that its composition may be modified later by the metabolic products of the foetus.

  2. The amniotic fluid has been found:

    • to increase relatively slowly in volume during early pregnancy;

    • to be somewhat hypotonic to the maternal body-fluids during the whole of its existence and at 46 days to have lower concentrations of creatinine and urea;

    • to contain concentrations of sodium and chloride similar to those in maternal or cord serum;

    • to contain concentrations of potassium approximating to those in the foetal serum.

We would like to thank Mr. R. W. Pomeroy for his never failing co-operation and Terry Cowan for his care in managing the animals.

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