414, ARTHUR ROBINSON. 
obliterated, and a new cavity, that of the archenteron, appears. 
The latter is from the first in communication with the exte- 
rior by a widely open orifice, the blastopore. In the majority 
of Vertebrates, however, the inner boundary of the didermic 
vesicle, the hypoblast, is not produced as a distinct layer of 
cells, but either as a solid mass of food-laden cells or food- 
laden protoplasm, which is not divided into cells. The former 
condition is met with in Amphibia (1), Cyclostomes (1, 14, 30, 
47), Ganoids (1); the latter in Teleosteans (1, 7), Elasmo- 
branchs (1); Lacertilians (50), Chelonians (1, 35), and 
Aves (1). 
The large hypoblastic mass in all these cases is produced 
by an increase of the protoplasm destined to form the ventral 
wall of the archenteron ; consequently the latter cavity, when 
it first appears, is situated excentrically. It les, however, 
within the hypoblast, and is therefore entirely separated from 
the segmentation cavity, which is represented either by a large 
cavity, which gradually disappears simultaneously with the 
increase of the archenteron, as in Elasmobranchs (1), Cyclo- 
stomes (1, 14, 30, 47), Ganoids (1),and Amphibians (1); or by 
a small and also transient cavity, as in birds (10) ; or from the 
first by a narrow space, as in the Lacertilians (22). 
The increase of the ventral hypoblast is associated with 
modifications, both in the formation of the archenteron and 
also in the completion of the didermic vesicle. 
In Amphibians the archenteron is produced by cleavage, 
which takes place excentrically in the hypoblastic mass (22 a, 
36,40). It commences peripherally at the posterior part of 
the ovum, and passes forward into its interior. The cavity is 
apparently formed in a very similar but somewhat modified 
way in Teleosteans. In Elasmobranchs (1), Cyclostomes (1, 
14, 30, 47), and Ganoids (1) the dorsal wall of the archenteron 
is said to be formed by embolic invagination, and its ventral 
wall by the hypertrophied ventral hypoblast, which appears 
during the segmentation. In all these cases, however, the 
archenteron commences before the inclusion of the hypoblast 
is completed, and there can be little doubt that the retardation 
