348 W. F. R. WELDON. 
mentation nucleus, but that it plays a part in the formation of 
the blastoderm.” 
The later history of this central mass is described as 
follows : 
“While the cells which have reached the surface and which 
have thus formed a blastoderm are undergoing division, this 
central protoplasm also divides and migrates, though much more 
slowly, to the surface,” giving rise to the blastoporic patch of 
cells described above. 
This account of the segmentation is so definite, and at the 
same time so inconsistent with anything which I have myself 
been able to observe, that I cannot offer any plausible 
suggestion as to the cause of the differences between Professor 
Kingsley and myself, unless it be that the mode of segmenta- 
tion in the same species really differs on the opposite shores of 
the Atlantic. 
The process of formation of the endoderm and mesoderm, 
which commences at the close of the period of segmenta- 
tion, proceeds so slowly that these layers are not completely 
established before the assumption of the Nauplius condition. 
The stages between the blastosphere already described and the 
perfect Nauplius will, therefore, be considered together. 
The changes in external appearance may be gathered 
from figs. 5—9. At the close of segmentation the egg is 
covered, as already stated, by a single layer of cells, which are 
uniform in character except over a small, depressed area, re- 
presenting the blastopore. This blastoporic area marks the 
posterior extremity of the future embryo, and that surface of 
the egg on which it lies is often slightly flattened, giving an 
indication of the ventral surface. The relation of the blasto- 
derm to the egg is, however, subject to change, owing to the 
readiness with which each egg is deformed by pressure. 
After the establishment of the blastoporic area, the cells of 
the ventral surface thicken, and those on each side of the 
middle line divide, so that an irregular band extends forwards 
from the blastopore on either side, in which the nuclei are more 
