GASTRULATION IN BIRDS. 
147 
edge is supported by experimeutal observations, sucli as 
marking certaiu parts of the blastoderm by injury, and 
following such marks through several hours of incubation. 
Although the account given forms a very complete story, 
which, if the observations are good, does seem to be 
supported by a good deal of evidence, yet it is very diffi- 
cult to reconcile it with the process of gastrulation in the 
other Amniota. In fact Patterson himself hardly mentions 
the I'eptiles or mammals, but confines his efforts to an attempt 
to adapt the bird to Amphibians and fishes, and more especially 
to the Teleostean fishes. That is to say, he tries to connect 
birds with a group far removed from them in anatomical 
features, and ignores the difficulties presented by his theory 
when compared with the most closely allied foi-ms. 
In no other group of vertebrates is the dorsal lip of the 
blastopoi’e, which is in every other case the most bulky and 
actively proliferating part of the embryo, known to exist as 
a thin or free edge ! It is extremely difficult to conceive of 
the mechanism by means of which snch an involution could 
take place. 
Again, on pp. 86-87, the author speaks of the whole thin 
edge as the dorsal lip of the blastopore, and the yolk as the 
ventral lip. Now this is never the case in any vertebrate, 
whether we consider the meroblastic eggs of the Elasmo- 
branch or Teleost or the less heavily yolked eggs of the 
Amphibia. In all cases the ventral lip of the blastopore, if 
fonned, is a thickened curved rim which is formed in con- 
junction with the inflection of the epiblast. If Patterson is 
i-ight in calling the inflected edge of the blastoderm the 
dorsal lip of the blastopore, then the part which he calls the 
ventral lip of the blastopore is surely the floor of the gut 
corresponding to the yolk-plug in Rana. The yolk is never 
the lip of the blastopore. 
Stronger evidence is the table given (p. 90) of measure- 
ments made upon liviug blastoderms during the time sup- 
posed to be taken for the process of gastrulation. If the 
edge of the ectoderm is inflected, one might expect to find a 
