1890.] Concrescence Theory of the Vertebrate Embryo. 2 
As regards the homologies of the layers, I consider that the 
outer layer of the vesicle is ectoderm, and the thickening which 
constitutes the embryonic shield corresponds to the ectodermal 
thickening of the embryonic area in Sauropsida; the inner layer 
and the lining of the “chorda-canal” (blastoporic) is the 
entoderm ; the remaining tissue of the primitive streak and head- 
process, together with the middle layer, constitute the mesoderm. 
In order not to prejudge the question, the names of the germ- 
layers have not been used in the preceding description of the 
blastodermic vesicle. 
Inversion of the Germ-Layers in Rodents—In many but not all 
rodents the outer layer, Rauber's Deckschicht, of the embryonic 
shield undergoes a remarkable hypertrophy immediately after the 
close of segmentation proper; the deckschicht, together with the 
ectoderm underlying it, becomes a plug which pushes in the other 
layers, thereby profoundly altering the topography of the ovum. 
In the mole, Heape, 23, the hypertrophy is not very great, and 
the plug disappears soon, so that there is no great change; in 
guinea pigs, mice, and Arvicola the plug becomes very large, and 
remains for a long time. The plug is very long, and the ovum 
elongates with it, changing into an almost cylindrical vesicle 
(Selenka's Keimcylinder). The plug becomes hollow, and the 
cells corresponding to the deckschicht become separated from 
those which are to form the ectoderm of the embryo. Three mod- 
ifications of the hollowing out of the plug and of the separation 
of its two parts are known. The changes referred to are very clearly 
illustrated by Selenka, 56, Taf. xvr., ina series of comparative dia- 
grammatic figures. In the simplest case, Fig. 21, the plug acquires 
a single cavity, 2; the cells around the upper end, D, correspond to 
the deckschicht, and serve partly to attach the ovum to the uterine 
walls ; the cells, £c, around the lower end of the .cavity become 
the embryonic ectoderm; all the cells around the cavity, a, 
are homologous with the outer layer of the embryonic shield of 
other mammals. The cavity, c, of the vesicle is very much re- 
duced; the inner side of the shield, z. e., of the plug, is lined by 
an inner layer, ez, which gives rise to the entoderm. The outer 
layer of the vesicle is very thin, and is found to unite very closely 
