1890.] Concrescence Theory of the Vertebrate Embryo. 627 
diately following stages there is a rapid expansion of the meso- 
derm, in all directions, and also an expansion of the archenteric 
cavity, which is especially noticeable in meroblastic ova. The 
relations of the blastopore are discussed below. 
2°. Mammalıan primitive streak—The description of the primi- 
tive streak has to be preceded by an account of the changes in 
the blastodermic vesicle from the close of segmentation to the 
appearance of the primitive streak. 
Mammalian blastodermic vesicle —After the close of segmenta- 
tion we find that the inner mass becomes flattened out, and in the 
region it occupies we can distinguish three layers of cells, as 
previously described, first, counting from the outside the thin 
layer of cells known as Rauber's “ Deckschicht;” second, a 
middle layer of cylindrical cells, which becomes the ectoderm ; 
third, an inmost layer of thin flattened cells, which belong to the 
entoderm; the “Deckschicht” continues round the whole 
vesicle as a single layer; the others layers do not so continue. 
The next step in development is the formation of a second 
layer which spreads out in all directions from the region of the 
inner mass; hence as far 
as the new layer reaches 
the blastodermic vesicle be- 
comes two-layered. Mean- 
while the “ Deckschicht ” 
disappears, leaving two lay- 
ers in the region of the inner 
mass; it is to be remarked 
that the “ Deckschicht ” is 
retained in certain rodents, 
undergoing special modifi- 
cation as described in the 
section on inversion of the 
er 16. Hecate pe vesicle of a rabbit rid germ-layers. The stage in 
ys. ag, area germinativa or embryon à : 2 
shel: gé, line, above which the vesicle is iac which the vesicle is partly 
= two-layered, while the 
“ Deckschicht” is still present, is exemplified in Fig. 16. 
Blastocyst of the rabbit of six days. —The development is ex- 
