a 
486 . The American Naturalist. Diy 
posed of entoblast that has not been invaginated. As in Amphioxus, 
this invaginated ectoblast forms the chorda and part of the mesoblast, — 
while the entoblast forms the rest of the mesoblast as well as the gut. 
Nearly the same process, according to Lwoff, takes place in the | 
Axolotls. But here the dorsal invagination begins, while the micro- 
meres are growing over the macromeres, and the ectoblastic rudiments H 
of the mesoblast never form part of the roof of the enteric cavity. n 
the frog, however, Morgan‘ and Robinson and Assheton? seem to have — 
found good evidence that the micromeres do not grow around the 
macromeres, but that the superficial ones of the latter are themselves 
gradually split up into micromeres, so if we are to accept Lwoff’s inter- 
pretation of the dorsal invagination, the gastrulation in this form may — 
better be described as being by delamination than by an epibolic ee 
cess. Moreover, according to the apparantly accurate work of R 
inson and Assheton, there is no invagination of the ectoblast in the ‘ 
frog. | 
In Selachians and Teleosts, Lwoff believes the entire entoblast arises 
from the yolk-elements, that is, the periblast. This is surprising in 
view of the many statements to the contrary, especially H. V. Wilson's 
work on the sea-bass. Lwoff finds the chorda and part of the meso 
blast to be formed from the same elements as the nervous system. The — 
entoblast also takes part in the formation of the mesoblast. 7 
In Lacerta, cleavage results in a two layered germinal dise. The — 
inner layer becomes the entoblast. An invagination from the outer — 
_ layer forms the chorda and part of the axial mesoblast. The rest of | a 
the axial and the peripheral mesoblast is of entoblastic origin. a 
The process in the formation of the germ layers throughout the — 
Vertebrates is practically the same as in Am phioxus, except that there i a 
is no invagination to form the entoblast, and there are, therefore, 20 
gastrula lips. There is but one point in the embryo that is homo- 
logous in all Vertebrates, and that is the point where the ectoblastie — 
invagination begins. The so-called neurenteric canal does not lie — 
between the neural canal and the archenteron, but between the former i 
and the chorda, and shows their intimate connection—R. P. B. y 
ny ATA E ae eae Oe: a ESAE SEA eT > IS A e EN ew A ETE E oe ee eee a a E A 
The Mantle of Ascidians,—In studying live specimens of 
larval Phallusia, A. Kowaleysky’ at first thought with Hertwig, Sem- 
* AMER. NAT., Aug., 1891, p- 753. 
“Quart. Jour. Mis. Sci., 32, 1891, p. 451. 
‘Bul. U. S. Fish. Com., 9, 1891, p. 209. 
7 Mem. l'Acad. Imp. Sci. St. Petersburg, 7, 37, No. 10, 1892. 
