O04 W. F. R. WELDON. 
doubted endoderm, and migrate towards the ventral surface of 
the yolk to form the mesodermic bands. 
The appearance of a section passing through the blastopore 
during this process of invagination is shown in fig. 14. The 
cells of the ectoderm, which cover the greater part of the sur- 
face of the embryo, are seen to be even more superficial than 
at the close of segmentation; and the divisions between them 
extend only for a very short distance into the yolk. The 
great mass of the yolk is therefore undivided; and into this 
undivided mass the ectoderm-cells wander. The figure is 
drawn with a considerable amount of care, and represents with 
fair accuracy the appearance of a typical section. It will, 1 
think, be admitted that there is no difference in character 
between the invaginated cells so great as to enable any 
observer to say that some are endodermic and others meso- 
dermic in nature. 
Immediately after the invagination of the ameeboid cells 
shown in fig. 14, the ventral surface of the blastoderm 
thickens, as is indicated by the crowding of the nuclei in 
fig. 5a. 
In embryos of the age of fig. 6, the endoderm is separated 
into two distinct portions: one of these is formed by the cells 
which were first invaginated (vitellophags of Nusbaum), and 
which have by this time become scattered irregularly through 
the substance of the yolk (figs. 15 and 16, En’); the other 
forms a more or less compact mass of cells, confined to the 
posterior portion of the embryo, and continuous with the cells 
of the persistent blastoporic area (fig. 15, Hn”). The posterior 
mass of endoderm frequently contains a small lumen, such as 
that shown in fig. 15; but this lumen is not always demon- 
strable, and I have never been able to show that it communi- 
cates with the exterior. 
The relations of the ectoderm at this stage will be under- 
stood from figs. 15 and 16, one of which represents a median, 
the other a lateral longitudinal section. The ectoderm is 
everywhere one cell deep ; in the middle ventral line (fig. 15) 
the cells are only slightly thicker than those outside the em- 
