FORMATION OF GERM-LAYERS IN CRANGON VULGARIS. 353 
Mysis. It is, however, quite incompatible with the account 
given by Kingsley, and already referred to, of the process in 
Crangon itself. The transverse arrangement of the nuclei is 
somewhat obscured in surface views by the peculiar curvature 
of the embryo; but it is, as will be seen, more evident in sec- 
tions during this stage than in younger embryos. 
From the stage represented in fig. 7 to the Nauplius stage a 
small patch of thickened ectoderm-cells—the “ dorsal organ ”’ 
—is present on the posterior dorsal surface of the embryo. I 
have nothing to add to the account of this structure which has 
been given by Kingsley. Its later history, together with that 
of other organs, I hope to describe in a future paper. 
The internal changes which accompany the development 
may now be described. 
Immediately after the formation of the blastoporic area 
shown in fig. 5, an invagination commences. ‘The cells of the 
blastoporic area divide, and apparently become ameceboid, some 
of them travelling from the surface of the blastoderm into the 
substance of the yolk. Certain of these cells, which evidently 
correspond to the ‘‘lower-layer cells” (vitellophags) of Nus- 
baum, send out processes in all directions among the sphe- 
rules of the yolk, and become irregularly distributed through 
its substance. Other cells remain, for a considerable time 
after their invagination, in the immediate neighbourhood of 
the blastopore. As will appear directly, I am inclined to 
regard the whole of the invaginated cells as forming endo- 
derm ; and I have failed to find any evidence of an ultimate 
formation of blood-corpuscles from the cells which first split 
off from the blastoderm. I have also failed to recognise the 
mesoderm cells spoken of by Kingsley, who distinguishes 
* some cells with large nuclei and ameceboid outlines, which 
are plainly budding out from cells at the mouth of the blasto- 
pore, and sinking into the yolk.” In Kingsley’s figure (first 
paper, fig. 9) there is no apparent evidence of any difference 
between the various cells which are undergoing invagination ; 
and no explanation is attempted of the manner in which these 
so-called mesoderm-cells separate from the rest of the un- 
