158 
UlCHAin) ASSIIETON. 
To me it seems that there is a very great difference between 
gastral and peristomal mesoblast : the one is protogenetic, the 
other deuterogenetic. 
Noi-j again, is it true that in the case of birds the whole 
mesoblast is formed from the primitive streak. It is altogether 
difficult to understand wli}' if the primitive streak is formed 
by the fusion of thickened rims, that thickening should dis- 
appear, only to reappear a little later as primitive streak. It 
is very remarkable and significant of the narrowness of this 
work that in dealing with avine early stages as compared 
with other vertebrates the word “ reptile ” should occur only 
twice and the mammal ” is mentioned but a single time ! 
It is pretty evident that the author is utterly unable to 
reconcile his description (which is an attempt to fit the birds 
on to fishes) with the facts of reptilian or mammalian embryo- 
logy, the two groups of animals most nearly connected with 
the birds. 
