434, A. A. W. HUBRECHT. 
Before passing from the nerve tissue to the muscular, men- 
tion must be made of the ulterior phases of the two epiblastic 
invaginations, which we noted in an earlier stage and about 
which we remarked that they would develope into the central 
cavity of the posterior brain-lobes. These ulterior phases have 
also already been described above, and we have only to add that 
the development of nerve-cells out of mesoblast cells which very 
soon surround the spherical sacs when they come to lie in the 
blastocel (1. c. (80), pl. i, figs. 39 and 40; pl. v, figs. 
74—80, 87), leads to the ultimate coalescence of the posterior 
with the anterior and superior lobes, and that in this phase 
hardly anything would denote the independent development 
and ulterior coalescence of the component parts.! The ques- 
tion whether the original epiblast cells, coating the interior 
cavity, indeed produce nothing but the epithelial lining of this 
cavity in the adult, or whether they might also contribute 
towards the formation of the nerve-cells by which this cavity is 
immediately enclosed, will always remain very difficult to answer 
with absolute certainty. For my own part, it will be obvious from 
what. I have remarked about the development of the nervous 
system in the mesoblast, that I should very much hesitate in 
accepting the latter view, that, on the contrary, I expect to 
find the nervous tissue which has a specific sensory nature— 
the epithelial lining of the cavity—and is epiblastic in origin, 
to be essentially distinct from that to which a conductive and 
perceptive significance must be accorded, the latter being of 
mesoblastic origin. 
Another component part of these posterior brain-lobes, viz. 
the accumulation of spherical refractive cells in their posterior 
portion, which has more than once been described in the adult 
(17, 20), was seen by me to develope in the tissue when it was 
already distinctly composed of nerve-cells and fibres. Certain 
1 Attention should here be drawn to a certain resemblance in the origin of 
these structures in the Nemertea and the origin of part of the brain-lobes in 
Mollusca (Dentalium, Pteropods) and Polyzoa as tubiform or vesicular inva- 
ginations of the epiblast according to the researches of Kowalevsky, Harmer, 
and Fol. 
