1. Females of an inbred CBA strain of mice, kept in the presence of males, were injected with c.c. of 1 per cent, trypan blue solution 7 days after the detection of a copulation plug. In one series of experiments the animals were killed and the embryos recovered 1,2,3, ‘c., days after the injection; in another series the females were allowed to bring their embryos to term.

  2. The injected dye soon becomes accumulated in the yolk-sac, and a little later is visible in the region of the placenta.

  3. In many -day embryos the rate of development has been slowed down; the closure of the neural tube is abnormal, and the tube itself may show bends and kinks. Sub-epidermal blebs may be found in various regions.

  4. The rolling up of the embryo into a right-handed spiral, which normally occurs at about days, is very often affected, the posterior part of the body being deflected to the right. At the same time, the pericardium begins to be very markedly inflated; and it seems probable that the abnormal turning of the embryo is caused by the mechanical impediment presented by the hypertrophied pericardium.

  5. There is often a considerable dilation of the embryonic blood-vessels, particularly in the head; and fluids tend to escape from the vessels. There are often haematomata in the tail region.

  6. There is a high death-rate of embryos at 12-13 days. Those which survive to term usually show few abnormalities other than shortening or kinking of the tail.

  7. There is a marked shortage of females in the surviving young. This is presumably due to a differential death-rate, and since many deaths occur by 12 days, the females must be more sensitive than the males at a still earlier period, well before the time at which sexual differentiation takes place.

  8. It is suggested that the main effects of the dye are firstly a general inhibition of development and secondly a more specific effect on the body fluids and circulatory organs.

1

Experiments on the effect of trypan blue on amphibian embryos, conducted while this paper was in the press, have shown that in those forms the dye suppresses the differentiation of neural structures and of the notochord. One must therefore keep in mind the possibility that the comparatively slight abnormalities of those organs seen in the mouse embryos may not be only secondary consequences of mechanical disturbance but may in part be produced by a direct action of the dye.

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