430 BASHFORD DEAN, 
The transition from the stage last described to that of the 
early gastrulation may first, however, be followed. In Pl. 30, 
fig. 10, is figured in surface view a late blastula; the light 
coloured dome-shaped blastoderm appears sharply marked off 
from the yolk ; its marginal rim, as a somewhat irregular line, 
seems as if in bold relief against a somewhat dark-coloured 
zone of the yolk; in surface view, in fact, it appears to 
overhang the yolk, and thus to represent the beginnings of 
gastrulation. That this observation, however, is incorrect is 
demonstrated both by sections and by a closer examination of 
the surface view of the object; the darker colour of the yolk 
zone is probably due to its merocyte-bearing character. It is 
in the stage figured in PI. 30, fig. 11, that gastrulation actually 
begins. The downgrowth of the blastoderm is accompanied 
by the slight overlapping of its rim at one side—at the left in 
the figure ; the remainder of the clearly defined margin has 
not as yet separated from the yolk. By the stage of Pl. 30, 
fig. 12, the downgrowth of the blastoderm has separated its 
entire rim from the yolk; and that portion which initiated 
the process of separation is now separated most widely as the 
dorsal lip of the blastopore. This region at a very similar 
stage has been shown in the following figure, Pl. 30, fig. 13, 
as exhibiting a variation! of considerable interest ; at the rim’s 
dorsalmost point a slight indentation is present, which may be 
supposed to correspond to the true blastopore of the ancestral 
Ganoid and Elasmobranch, the remaining portion of the rim 
representing the circumcrescence margin (O. Hertwig). 
Later surface views of the gastrula are seen in Pl. 30, figs. 
14,15. In the former the continued growth of the blasto- 
derm has greatly reduced the size of the blastopore; this now 
appears as a circular opening, its margin slightly nicked on 
both ventral and dorsal sides, and shows dorsally the whitened 
tract which marks the appearance of the embryo. In the 
1 Variations in the outline of the closing blastopore are not uncommon 
(cf. Lepidosteus, Dean, op. cit., p. 22). Oval blastopores were noted: in 
some one or both of the marginal indentations had disappeared; the dorsal 
one, however, is usually persistent. 
