512 The American Naturalist. [June, 
demonstrated in C. Kuppfer’s figures, 36, Taf. vur., Figs. 1-3. 
Moreover, Kuppfer and Benecke have found, in certain cases, the 
so-called “ Sichel,” or transverse thickening at the hind end of the 
growing primitive streak ; this thickening is a portion of the blas- 
todermic rim, and by the interpretation I adopt until better obser- 
vations shall decide, it corresponds to the transverse thickening 
in a similar position in sharks. 
The process of concrescence in birds was partly indicated 
by Koller’s investigations, 30, 3z, and has been carefully 
elucidated by Duval, 78, 1. The resemblance to concrescence 
as known in elasmobranches is very striking. Around the 
edge of the blastoderm appears very early a small groove; 
as the blastoderm expands the front portion loses the groove; 
one point, the centre of the groove, ceases to move, or 
at least moves much more slowly than the remainder of the blas- 
todermic rim; as the expansion continues the edges of the two 
halves of the groove coalesce gradually behind the fixed point, 
thus producing the entodermal canal in the same manner as in 
sharks; cells accumulate at the same time and make the so- 
called primitive streak; most of these cells enter into the com- 
position of the mesoderm. There is an uncertainty in Duval’s 
account owing to his failure to distinguish between the segmen- 
tation and the entodermal cavities; as I have pointed out, 
AMERICAN NATURALIST, 1889, p. 475, Duval’s subgerminal-cavity 
at the close of segmentation is the true segmentation cavity (blasto- 
coele); his own account of the concrescence shows that the ento- 
dermal canal arises separately and behind the blastodermic or 
true segmentation cavity ; but unfortunately neither his text nor 
his figures give satisfactory information in regard to the separa- 
tion of the two cavities; Duval calls the entodermal cavity also 
the subgerminal, thus causing confusion, since subgerminal is 
properly applied only to the blastoccele. 
_ In birds (hen’s ova) there is a further peculiarity, which is, I 
think, probably to be found in all amniota, namely:—that por- 
tion of the edge of the ectoderm which does not share in con- 
crescence, and which corresponds to the edge of the anus of 
Rusconi, closes over the yolk behind the primitive streak, so that 
