THE DEVELOPMENT OF THE CAPE CEPHALODISCUS. 197 
An interesting result of the rapid increase of these yolk 
columns is that an invagination is formed near the point 
(PL 13, fig. 15, p. inv.) where they are attached to the inner 
mass, apparently due merely to mechanical causes associated 
Avith the increase of the outer layer. This gives the 
appearance of a gastrula-like structure, which, however, can 
only be fully discussed when the changes in the inner yolk 
mass are considered. 
Origin of Ectoderm. 
After the formation of the external yolk columns, their 
cells divide rapidly, and the dermarcation between them, so 
clearly marked before, disappears. In view of their origin, 
Ave must regard each of the yolk columns as representing a 
single cell, and the breaking down of the cellular structure 
as due probably to the great abundance of yolk, the cell 
having lost control of the elongate and attenuated column. 
What Avas observed to occur at this stage was that the 
multiplying nuclei, with a certain amount of protoplasm, 
became each closely applied to a yolk granule, the two 
forming an ovoid body, in which the yolk granule, deeply 
stained Avith eosin, could be clearly distinguished from the 
nucleus, which Avas as distinctly stained with hsematoxylin. 
Presumably the yolk granules serve as nourishment for the 
rapidly multiplying cells, for, ultimately, the yolk granules dis- 
appear, first from the peripheral parts, and later from the 
deeper parts of the ectoderm, till finally only a network of 
protoplasm, or rather a vacuolated protoplasmic mass, with 
numerous nuclei embedded in its substance, is left. 
Two other points may be noted in connection with the 
origin of the ectoderm, viz. the formation of a basemeut 
membrane and the occurrence of excretory matter. With 
regard to the first, the outer cells were always distinguishable 
from the inner, except posteriorly, and an intervening space 
is seen in preparations of the more advanced stages. This 
demarcation becomes more distinct by the appearance of a 
