ABSTRACT
On p. 945 of the first volume of his ‘Handbuch der Entwickelungslehre ‘Oscar Hertwig refers (in a postscript to his theory of the germinal layers) to a theoretical view concerning the process of gastrulation in mammals (and also in vertebrates generally) which I have attempted to establish in my article on ‘Keimblattbildung und Furchung bei Tarsius spectrum’ (1902, Verhandel. Kon. Akad. v. Wetensch. Amsterdam). He points out that I have “considerably changed my views,’ and that I have “now been induced to look upon matters in a way which differs considerably from what lie (Hertwig) and many other embryologists are ready to uphold.”
In this case the blastococl gives rise quite naturally to the archenteron.
“On the Invaginate Planilla,” etc., ‘Quart. Journ. of Mier. Sci.,’ vol. xv, 1875, p. 163.
Lwoif, “Die Bildung der primaren Keimblâtter und die Eutstehung der Chorda,” etc., ‘Bull Soc. imp. der naturalistes de Moscou,’ 1S94.
I must emphatically assert that the reproach of having included the invagination process in the definition of gastrulation does not apply to Keibel himself, however much this might seem to be the case if we consider his definition No. 1. He has, however, on pp. 1109-1110 of the ‘Referat’ above alluded to, most distinctly stated that he wished this definition to apply to a delamination-gastrula as well.
In Haeckel’s ‘Anthropogenie ‘(4th ed., 1891, p. 156), which may also be considered as decisive on this question, we read: “[The cleavage cells] arrange themselves in two separate layers, the two primary germinal layers. These surround a digestive cavity, the ‘Urdarm’ (primitive enteron), with an opening, the ‘Urmund ‘(primitive mouth). The important embryonic stage which possesses these oldest primitive organs we call the gastrula, the process by which it originates gastrulation.”
‘Anat. Anzeiger,’ iii, 1888, p. 911; ‘Quart. Journ. of Mier. Science,’ vol. xxxi, 1890, p. 552.
“Zur Entwickelungsgeschichte der Chorda, etc.,” ‘Archiv f. Anat. u. Physiol., Anat. Abth.,’ 1889, p. 376.
An open blastopore has up to now only rarely been noticed in Mammals. In ErinaceusI Lave observed it quite clearly (‘Furchung und Keimblattbildung bei Tarsius,’ Pl. XII, figs. 8, 9). Keibel (l.c. Taf. 24, figs. 46, 47) is less positive as far as the rabbit goes, so are Selenka (Taf. 17 and 18) for the opossum, and Bonnet (Anat. Hefle, Bd. 9, Taf. 32) for the dog.
* Furchung and Keimblattbildung bei Tarsius,’ Amsterdam, 1902, K. Akademie v. Wetensch.
Whether these arrangements are repeated in higher Vertebrates has not yet been decisively shown. It should, however, be noticed that the mode of formation of the pericardium which was two years ago described by me for Tarsius (1. c., p. 3, El. VIII, fig. 70 a, b), is in no way isolated, butlias since been also observed by me in Sciurus and Tupaja. For Tupaja the gradual transition of an évagination of the entoderm that becomes constricted off into the anterior median portion of the pericardium is demonstrated in more than one preparation. In Scinrus it has not been followed out in full, but an early stage was noticed. Bats, too, seem promising in this respect. At all events, if this should be further confirmed, our whole interpretation of the vertebrate cœlom would have to be recast. That in such a case Balanoglossus among Invertebrates would have to be considered as an object of coinparison would not be astonishing if we remember how a certain comparability between the branchial arrangement of Balanoglossus and Amphioxus has long been known. The so-called notochord of Balanoglossus I would be inclined with Spengel to regard as a delusion. Gegenbaur in the latest edition of his comparative anatomy (Bd. 1, p. 185) has allotted to Rhabdopleura, which is related to Balanoglossus, a decided significance in the pedigree of Vertebrates.
