756 
The American Naturalist. 
outer layers enter into the formation of the ectoderm, while the 
inner layer is concerned in the production of the permanent 
entoderm ; the outermost layer Rauber terms the Deckschicht 
Lieberkiihn j/rt, and others have since then confirmed Rauber's 
Homologies of the mammalian blastodermic vesicle. — We have 
so little accurate information concerning the details of the forma- 
tion of the blastodermic vesicle, that any interpretation must be 
tentative. I still consider, however, the view which I brought 
for^vard in 1885 {Buck's Reference Hdbk. Med. Sciences, I., 528), 
as the most satisfactoiy, and preferable to the similar explanation 
advanced independently and simultaneously by Haddon, 20, and 
reproduced by him briefly in his " Practical Embryology," 47-48. 
F. Keibel, 27, advocated similar interpretations two years later, 
but without quoting Minot or Haddon. I regard the sub- 
zonal epithelium as the entoderm, and the inner mass of cells as 
the primitive blastoderm or ectoderm : by so doing the parts can 
be readily and exactly homologized with the parts 
be evident 
mammalian vesicle be com- 
pared with the section of a 
segmented amphibian 
ovum, (Fig. 4). 
The primitive blastoderm 
Bl. or ectoderm consists of 
several layers of cells rich 
in protoplasm ; below it is 
the large segmentation cav- 
ity, S.C., relatively much 
larger in the mammalian 
than in the amphibian ovum. 
At Its edge the primitive 
blastoderm joins the ento- 
derm yolk, which in am- 
phibia is a large mass, in 
mammals only a single layer of cells. Now 
ancestors of the higher mammals had ova wi 
if the diagram (Fig. 14) of the 
ng theTemnanf of^egmen 
