1890.] Concrescence Theory of the Vertebrate Embryo. 513 
the portion of the yolk which is left uncovered is remote from 
the embryonic area (or primitive streak). Asa rare anomaly (see 
Whitman, 68), a line is visible running in the ectoderm from the 
hind end of the primitive streak to the edge of the uncovered 
yolk; this line is to be interpreted as evidence of the growing 
together of the ectoderm behind the streak proper. The ecto- 
derm, as it spreads over the yolk, receives no accretions from it, 
but accomplishes its expansion by proliferation of its own cells. 
Thus the uncovered yolk is bounded by the free edge of the 
ectoderm; this area, which may be called the yolk blastopore, 
is comparable to the anus of Rusconi, from which it differs in 
position, being remote from, instead of close (as in the anus of 
Rusconi) to the primitive streak, 
for it is situated nearly opposite 
the embryonic area. In birds, 
according to Duval, 79, the 
yolk blastopore (Dotternabel) is 
never closed by ectoderm, but 
remains covered by the auner 
membrane only until the meso- 
derm spreads over it. The 
growing edge of the ectoderm is somewhat thickened; it finally 
is reflected around the edge of the yolk blastopore, forming as 
it were a funnel, at the bottom of which is the yolk. (See 
Duval, $ ¢.) 
Concrescence in Mammals.—tThere is, as yet, no direct evidence 
of concrescence in mammals, but strong indirect evidence. First, 
it is to be remarked that of no mammal do we know the exact 
history of the primitive streak, and secondly that what we do 
know accords fully with the history in the Sauropsida. There is 
a primitive blastoderm on the edge of which the primitive streak 
appears, and from its beginning the streak grows backwards as 
the germinative area expands. Now the mammals are derived 
from the reptiles, and it may be assumed safely that their early 
development is essentially the same as that of the reptiles, and 
+ what koewi we pe agrees with this inference. Unfortu- 
5 Du 3 3, th hilic ombilical 
TEN 

after Duval. 


