22 GASTRULATION OF VERTEBRATES. 



Amphioxus amongst vertebrates, 1 and in many invertebrates with holoblastic (alecithal) ova, 

 the bilaminar blastoderm is produced, not by delamination, but by the invagination of one 

 pole of an originally simple hollow spherical blastodermic vesicle, the invaginated portion 

 becoming the primitive entoderm and the remaining part of the wall of the vesicle forming 

 the primitive ectoderm (fig. 24). This condition, which was discovered by Kowalewsky, is 

 known as the gastrula stage, and it is regarded by most embryologists, following Haeckel, as 

 typical of the mode of formation of the bilaminar blastoderm throughout the animal kingdom. 

 The aperture of in vagina tion by which the cavity of the entoderm communicates for a time 

 with the exterior has been termed the Wastopore (Lankester). 



It is not possible in this account of the embryology of the mammal (which must necessarily 

 be very short) to examine at any length the evidence upon which the opinion rests that a 

 gastrula stage can be shown to exist at an early stage in the development of the meroblastic 

 ova of the lower vertebrata. It will be sufficient for the present purpose to state that in 

 fishes, reptiles, and birds, the ova of all of which are of a markedly meroblastic type, that 

 part of the ovum in which alone segmentation has occurred, and in which active development 

 subsequently proceeds, produces a bilaminar blastoderm as in the mammal by the separation off 

 as a distinct layer of a lower or inner stratum of cells to form the primitive entoderm. 

 whilst the remaining cells arrange themselves into an upper or outer stratum, the primitive 

 ectoderm. 2 At one part of the circular blastoderm which has thus been formed there now 

 occurs a crescentic thickening of the ectoderm, on the surface of which a pit or depression 

 becomes formed by an invagination of the ectoderm. This pit extends inwards until it abuts 

 against a subjacent entodermal thickening, and it may even penetrate the entoderm and 

 communicate with the cavity below the blastoderm (which afterwards becomes in part con- 

 verted into the posterior end of the alimentary canal). The invagination in question has been 

 regarded as a rudimentary blastopore, its time of formation having become shifted to a later 

 period, and the entoderm having already been formed by delamination altogether independently 

 of, in place of resulting from, the invagination. as in the typical mode of gastrula formation. 



In the mammal a similar invagination of the ectoderm also occurs at the posterior extremity 

 of the embryonic area, and this invagination has been described by Heape in the mole as 

 communicating for a time with the cavity of the blastodermic vesicle (fig. 22, blpi), which sub- 



^33S?f?SS3?^ Fig. 25. SURFACE VIEW OF AM EMBRYONIC AREA OF THE MOLE 



X^f-' '^X IN WHICH THE MEDULLARY GROOVE HAS BEGUN TO APPEAR 



^B Bk IN FRONT OF THE PRIMITIVE STREAK. AT THE JUNCTION OF 



%V |&A THE TWO A SMALL APERTURE IS SEEN : THIS IS THE DORSAL 



J|- ^\ OPENING OF THE OBLIQUE NEURENTERIC CANAL. (Heape.) 



tHI se( l 11 en tlj becomes converted in part into the alimentary canal. 

 In birds and reptiles as well as mammals the invagination in 

 question soon becomes extended forward along the middle line 

 of the blastoderm as a linear groove (primitive groove), which 

 indents an ectodermal thickening (primitive streak), and if 

 the posterior invagination represents a blastopore. this 

 ^jj groove must be looked upon as an extension of such blas- 



^H <:^ topore, a view which derives support from the fact that 



there appears to be a tendency for the primitive groove, at 

 least its anterior end. to penetrate to the entoderm, and thus 

 to form here also a canal of communication between the cavity 

 below the entoderm and the exterior. Such a canal is desig- 

 nated " neurenteric." because the anterior end of the primitive streak and groove becomes 

 eventually enclosed by the neural tube, and the canal then effects a (temporary) communica- 

 tion between the neural tube and the enteric canal. 



Another important point of resemblance between this invagination and the blastopore of 

 the typical gastrula is the fact that the middle layer of the trilaminar blastoderm begins to 

 develope from the margins of the invagination. But in this respect again there is a differ- 

 ence, for whereas in the simplest and most typical forms, such as Sagitta amongst invertebrates, 

 and Amphioxus amongst vertebrates, the middle layer (mesoblast) originates as a pair of hollow 

 protrusions of the primitive entoderm (ccelom-invaginations of Hertwig, figs. 28, 29) ; in 

 mammals and birds it makes its first appearance in the form of solid outgrowths from the 

 primitive streak. 3 



Other views concerning- the g-astrulation of vertebrates. Kupffer regards the part 



1 Also, according to Hofmann, to some extent in elasmobranch fishes. 



2 No layer corresponding with Rauber's layer of the mammal is known to exist in lower vertebrates, 

 unless that layer is to be regarded as the homologue of the external (corneous) stratum of the epiblast, 

 which is found at a later stage in fishes and amphibia. 



3 Ruckert has described an imperfect form of coelom-invagination in elasmobranch fishes, and 

 Hertwig in amphibia. 



