» 
_ of the blastopore in the Amniota gastrula as 
1891.] Embryology. 167 
our attention to that part of it dealing with the gastrulation of the 
vertebrates. The Selachian gastrulation arose by the accumulation of 
yolk in the cyclostome egg, while the Amniote (reptiles, birds, mam- 
mals) gastrula arose from accumulated yolk to the amphibian egg. 
The resulting gastrulee of Selachians and Amniota, the author attempts 
to show, are therefore fundamentally different. The Selachian (and 
Teleost) gastrulz resulted from the addition of yolk to the endoderm cells 
of the cyclostome before the ectoderm had grown over the endoderm, 
and since the epibolic endoderm does not cover in the yolk, the blasto- 
pore in this group is represented by the whole margin of the embryonic 
shield. The blastopore mouth then is very large, and the (morpholog- 
ical) posterior end of the blastopore lies just in front of the embryonic 
shield, and the anterior or upper end of the blastopore lies at its 
usual position at the posterior end of the shield. This is, of course, the 
general conception. But for the Amniota the author believes the 
gastrula to be different in that it is not here represented by the whole 
border of the embryonic shield, but has a more limited extent. Rabl 
believes that the accumulation of yolk in the amphibian egg has been 
also in the endoderm cells, but, so far as he explains it, this must have, 
taken place after (ancestrally) the epiblast had covered the (endoderm) 
yolk-mass so that the gastrula becomes reduced to the region of the 
primitive streak alone. Therefore it follows that one end of the 
primitive groove (just behind the embryonic shield) represents the 
anterior (upper) end of the amphibian blastopore, and the other end 
of the groove the posterior (lower of the amphibian). The anterior 
end of the primitive shield would not seem here to represent anything 
in particular! Rabl supports his conclusion by arguments drawn from 
the formation of the mesoderm. 
The author does not account for- the large exposure of yolk outside 
we find it in the bird and 
lizard ; unless indeed he supposed it to. have actually broken through 
the ectoderm covering. Further, that the author’s view is probably 
erroneous is shown in the occasional presence of a lengthened primi- 
tive streak running posteriorly through the area opaca, as — 
figured by Whitman. It has also, I believe, been seen since by others. 
