802 MR EDWARD J. BLES ON THE 
than the neighbouring epiblast cells. This difference in tint is found, by watching 
the cells in a living egg, to be the sign of an approaching division of the pale cell. 
The pigment reappears apparently undiminished in quantity after division. The cause — 
of such cells turning pale at this particular time I hope to discuss later, as there 
appears to be some parallel between this process and that of contraction in the 
chromatophores. 
Gastrulation.—A stage of the development of the blastopore is shown in figs. 6 
and 6a. In the side view (fig. 6), the egg is shown in the vitellme membrane and 
lying with its orientation as in life, the lowest point on the egg sphere being 30° 
behind the middle of the dorsal lip of the blastopore. This view shows very clearly 
features not easily shown in the ventral] view, namely, the extent of the depression of 
the posterior lip of the blastopore and the sharp, slightly puckered edge of the dorsal 
lip, which is not depressed, but remains up to the edge coincident with the surface 
of the sphere of the egg. At this stage indications are already noticed of the 
arrangement of ectoderm cells in rows, forming alternating light and dark streaks lying 
in the position of great circles roughly coaxial with the great circle passing through 
the longitudinal axis of the embryonic area. These streaks are indicated in fig. 6, 
passing from the posterior edge of the upper pigmented area towards the blastopore. 
In fig. 6a, the lower ends of these streaks are seen near the right-hand angle 
of the blastopore. In this figure (6a) the blastopore has reached the greatest lateral 
extension it attains as a crescentic opening. 
In fig. 7 the ege is seen from behind, and a stage is represented where the blastopore 
has just become circular. The yolk plug does not protude; at no time does it become 
prominent in Xenopus. The pigmented cell-area now extends back to the edge of the 
upper lip of the blastopore. A peculiar feature of this stage is the constant occurrence 
of alternating bands of darker and lighter cells arranged as described above in the previous 
stage figured (fig. 6). The observation that epiblast cells undergoing mitotic changes 
become pale at certain stages of the process leads to the interesting conclusion that the 
lighter patches are areas in which the epiblast cells are proliferating simultaneously. 
The Embryo.—AIn figs. 8 and 8a, representing views of the same egg seen from behind 
and from before, an early stage of the development of the medullary folds is shown. The 
view from behind (fig. 8) shows that the medullary folds are externally much less obvious 
at the hinder end than they are in the head region seen in fig. 8a. On the dorsal contour 
of the latter figure, the gentle elevations are due to the swollen medullary folds, and the 
slight notch between them indicates the neural groove (Liickenrimne). The neural groove 
extends far forwards, as far as the anterior wall of the brain, and can be seen between 
the paired dark patches on the exposed floor of the thalamencephalon. These areas 
are occupied by the pigmented cells of the optic vesicles, and closely resemble those 
described by EycLesHEIMER in Rana pabustris (93). Returning to the medullary folds, it 
will be seen from fig. 8a that at about the middle of their length the medullary folds 
have converged from the anterior end to lie side by side. It is thus possible to identify 
