318 
E. W. MACBRIDE. 
Ainpliioxus, is here strongly marked. This difference goes 
on accentuating itself as we ascend in the Vertebrate scale, and 
it is, of course, this difference which gave rise to the idea that 
notochord and mesoderm were ectodermic invaginations. The 
reason of this difference is not far to seek. From the dorsal 
wall of the archenteron notochord and mesoderm 
are formed by processes of folding or by proliferation. 
Now, for either of these two processes yolky cells are pre- 
eminently un suited, and hence there is an advantage in 
storing the yolk necessaiy for development in a portion of 
the archenteric wall where formative processes do not occur 
until very late. 
The work of Kerr (19) on the early stages in the develop- 
ment of Lepidosiren is based on a great quantity of material, 
and his figures and the general trend of his conclusions 
support those of Brachet. I call attention in particular to 
the emphasis he lays on the fact that the archenteric cavity 
first appears within the region occupied by the endoderm 
cells, and on the fact that the ventral lip of the ectoderm is 
“ cut out ” from the surface of the large cells of the ventral 
half of the egg — that hei’e, as in tlie dorsal lip, growth with 
differentiation goes on. 
The work of Bashford Dean on the embryology of the three 
Ganoid fish, Acipenser (11), Lepidosteus (11), and Amia 
(12), enables us to pass gradually from a development like that 
of the frog to that of a meroblastic egg. In Acipenser (vide 
text-fig. 5) the whole egg is segmented superficially. Above 
the whole mass is divided into cells, and there is a spacious 
segmentation cavity below ; the segmentation is superficial, 
and there is an interior mass of nnsegmented yolk. In Lepi- 
dosteus there ai’e but a few superficial furrows at the lower 
pole which subsequently completely disappear, whilst in Amia 
no furrows are formed at the lower pole at all. In all three 
cases at a certain point, which afterwards forms the dorsal 
lip of the blastopore, growth and differentiation of cells occur 
accompanied by invagination of some of the newly-formed 
cells. The ventral lip is formed in the same way, and grows 
