1890.] Concrescence Theory of the Vertebrate Embryo. 709 
den’s homology seems to me utterly baseless, The relations of 
the head-process in the sheep are very much as in the rabbit, 
Bonnet, 77, 65-67 ; the cells of the middle layer are at first free 
as they grow forward to form the process, but subsequently are 
found united with the inner layer. 
The head-process (cf. Lieberkühn, 43) probably always grows, 
as is certainly the case in the guinea pig, at its hinder end and at 
the expense of the primitive streak; it is, I think, in this manner 
that the often-noticed shortening and final disappearance of the 
streak is effected. The back growth of the process establishes 
the necessary condition for the growth of the notochord at its 
hind end. 
Homologies of the Mammalian Blastocyst.—There is at present 
no satisfactory and generally accepted interpretation of the parts 
of the mammalian blastocyst as compared with the corresponding 
stages of other vertebrates. The principal difficulties are two, 
namely: 1, the development of the two-layered stage; 2, the 
identification of archenteric cavity. 
I. The two-layered stage is said to develop by an zuzer layer 
growing out in all directions from the inner mass of cells left at 
the close of segmentation. Now we must look on this statement 
with great suspicion, because in a// other vertebrates it is the ecto- 
derm which grows over the ovum; it is therefore improbable 
that in mammals it is the entoderm; and, in fact, I cannot find 
anywhere any definite observations to show that it is the inner 
layer the spreading of which renders the blastodermic vesicle 
two-layered. If the current statement proves erroneous, then we 
shall gain much towards a direct comparison of mammalian de- 
velopment with that of other vertebrates. 
2. The permanent archenteric cavity arises from two sources, 
namely: the large space of the vesicle enclosed by the inner 
layer, and secondly, the so-called chorda-canal of the Kopffort- 
satz. Concerning the homologies of the latter, three views have 
been advanced: 1, it is the homologue of the tubular noto- 
chord; 2, it is the true archenteric cavity (E. Van Beneden, 3); 
3, it is the blastoporic canal (Minot, 1887, Buck’s Reference 
Handb., VL, p. 247). The third view is the one which I adopt. 
