538 PROCEEDINGS OF THE ACADEMY OF [DeC, 



and 4c are shown in dotted outline. Just posterior to the nucleus of 

 46 lie the two small cells 4c/^-^-^ and 4cp-^-^ with the darker staining 

 nuclei. Just posterior to these cells are the slightly larger cells 4d^-^-^ ' 

 and 4d^-^-^-^ mentioned above. Finally, extending dorsally and anter- 

 iorly from each of these are two cells wiiich represent the results of the 

 division just mentioned and for which the spindles are shown in figure 

 34. The resemblance to the so-called "mesoblast bands" of annelids 

 and mollusks is, I think, evident. In many cases, however, the 

 "band" formation is not so marked as in this egg, which was chosen 

 for drawing on account of its regularity. In many cases the meso- 



P.S.c 



■mac 

 Fig. 2. — Schematic optical section of an egg viewed from the posterior side. To 

 show the relation of the ectoderm, mesoderm and endoderm. Mes. 1, Meso- 

 derm derived from 4d^. Mes. 2, Mesoderm derived from second quartet 

 cells. JS'nd., Endoderm from 4d'. e, Entoblasts from 4d^. p. gr. c. Primitive 

 ganghon cells. 



blast cells do not form such evident bands. Instead the nuclear 

 spindles in the cells 4d^-^-^-^ and 4(^2-2.2.2 ]jg g^^ varying angles. This is 

 shown by the direction of the spindles in these cells in fig. 34. The 

 result tends to be a cluster of cells rather than symmetrical bands. 

 The lineage of these cells becomes too involved to trace further with 

 any accuracy. They lie well towards the dorsal side of the egg, and 

 become more or less confused with the cells which are now budding 

 in from the first quartet to form the ganglion. Nevertheless a fairly 

 definite group of cells can be found in this region during several of the 

 succeeding stages. I have attempted to represent something of the 



