1893.] Embryology. 485 
EMBRYOLOGY.’ 
Germ-layers of Vertebrates.’—Basilius Lwoff has extended 
his work upon the germ-layers of Amphioxus’ to other vertebrates and 
put forth a preliminary paper that arouses in us a lively interest in the 
final detailed account that is to contain both his own discoveries and a 
discussion of the literature of the subject. 
In the present paper he makes statements directly contrary to some 
Opinions that up to the present time we have supposed to be well- 
founded in fact. 
Besides Amphioxus, he has studied the formation of the germ- 
layers in Petromyzon, the Axolotl, Pristiurus, Torpedo, Labrax, Julis, 
Gobius, and Lacerta. He regards the Chordata as derived from a gas- 
trula-like ancestor. Gastrulation is defined as the process by which 
the gut is formed, and the entoblastic cells are those which form the 
ut, whatever else they may produce. In Amphioxus and Verte- 
brates with holoblastic eggs, cleavage results in a blastula, one-half of 
which is composed of micromeres, the other of macromeres. Owing to 
the continued more rapid multiplication of the micromeres, they cover 
and grow around the macromeres. When the blastula is single 
layered, as in Amphioxus, this results in the invagination of the macro- 
meres, where it is several Jayered, as in Petromyzon and the Amphibia, 
they are simply surrounded by the micromeres. 
The macromeres form the gut and are therefore entoblast, while the 
micromeres form the outer covering and are therefore ectoblast. But 
there can be no distinction between entoblast and ectoblast until the 
micromeres have surrounded the macromeres. Besides this process of 
gastrulation there is a dorsal invagination that has nothing to do with 
the formation of the gut, but forms the whole ectoblastic rudiment of 
the chorda and the mesoblast. 
The blastula of Petromyzon contains an extensive cleavage cavity, 
the roof of which is composed of micromeres and the floor of macro- 
meres. The former multiplying more rapidly, grow around the latter. 
At the same time a new cavity is sunk into one side of the embryo, 
the roof being formed of invaginated ectoblast, while the floor is com- 
1This department is edited by E. A. Andrews, Johns Hopkins University. 
* Biol. Centblt. 13, 1893, 40-50, 76-81. 
3 See AMERICAN NATURALIST, March, 1893, p. 228. 
