436 BASHFORD DEAN. 
2. The segmentation cavity tends to become flattened and 
to recede centrad. 
3. The embryo tends to become larger, and its structures 
more precociously differentiated, concentrating its substance 
more and more early in the sagittal plane of the dorsal lip 
(and leaving, therefore, the remaining region—e. g. ventral 
lip undifferentiated), and growing notably in the head region. 
4, The embryo, pari passu with its more precocious de- 
velopment, has come to acquire more perfect relations with 
the yolk. The growth of the embryo in the generalised con- 
dition is derived from deep merocyte layers of the yolk ; in the 
more specialised conditions, however, the more peripheral 
layers of deutoplasm become of service. Cellular increment 
is derived in Amia from a double tier of cellular zone of the 
floor of the segmentation cavity, which in the Teleost becomes 
clearly homologous with the periblast. In such an event the 
celenteron becomes clearly confluent with the segmentation 
cavity. 
From the foregoing discussion the writer’s views as to the 
homologies of the structures of the Teleostean gastrula are 
clearly apparent; and they will be found to correspond with 
those which he had formerly expressed in his paper on the 
gar-pike and sturgeon (p. 52). His interpretation requires, 
accordingly, the Ziegler-Wilson conception of the gastrula to 
become modified as follows:'—The ccelenteron extends under 
the rim of the blastoderm, from the free end of the “ primitive 
hypoblast ” to that of the “ ventral mesoderm ” of H. V. Wilson, 
the periblast yolk losing its ancestral cellular connection with 
the embryo, on account of acquiring its indirect but more 
highly important (and specialised) method of furnishing its 
cellular increments. 
F. The Mode of Formation of the Embryo.—The 
embryo has already made its appearance by the time of the 
closure of the blastopore. It is there recognised in the flat- 
tened and opaque cell mass extending in front of the blastopore, 
and enclosing about 180° of the egg’s circumference (Pl. 30, 
1H. V. Wilson, op. cit., p. 264, 
