532 PROCEEDINGS OF THE ACADEMY OF [DeC,. 



indications of covering over these cells. I believe that these apical 

 cells form a part of the ectodermal covering of this region of the 

 embryo. And furthermore I believe that these are the first of a 

 series of divisions occurring from now on in all the ectodermal cells, by 

 which a small epithelial cell is cut off towards the exterior and a larger 

 cell remains somewhat deeper down. It is l^y this kind of divisions 

 that nearly all of the later ectodermal layer is formed. These divisions 

 will be discussed farther on in this paper. 



Whether these apical cells form a definite sense organ or not I am 

 unable to say. It is in this region, as Lang points out, that the eyes 

 arise and that later the tentacles of the adult appear. However, the 

 exact fate of these cells is mere speculation, since I have found it 

 impossible to distinguish them in the late segmentation. 



Further History of First Quartet. 



We have so far followed the divisions of the first quartet until it is 

 composed of twenty cells, and to the time when there are forty cells in 

 the embryo. The last cells to be formed were the small apical cells, 

 la^-^-^-ld^-^-^. Very soon after this the cells la^'^-ld^-^ divide in a 

 dexiotropic manner into two almost equal cells. The lower cells, 

 la^-^-^-ld}-^"^, appear slightly smaller than the upper cells (figs. 21, 22). 

 Next, and sometimes coincident with the last division, spindles appear 

 in the large cells la^-^-^-ld}-^-^. These divide in a dexiotropic direction 

 into two very unequal cells (figs. 22, 23). The upper cells, la^-^-^-^- 

 Id}-^-^-^, are much the smaller and lie on the surface of the egg between 

 the cells la^-^-^-^-ld^-^-^-^ (fig. 24). While this division is occurring the 

 cells la^-^-ld^-^ are dividing in a laeotropic direction (figs. 22, 23). 

 Shortly after these divisions are completed the cells la^-^-ld^-^ divide 

 dexiotropically (fig. 24). In this case the lower cells, la'-^-^~ld^-^-, 

 form very small cells which lie on the surface of the egg (figs. 24, 27). 



At this time there are thirty-six cells in the first quartet and about 

 sixty-six cells in the entire embryo. This fact indicates clearly the 

 very rapid development of the upper hemisphere. In Crepidula at 

 the sixty-eight-cell stage there are only sixteen cells in the first quartet. 

 In Fiona at a similar stage there are twenty cells in this quartet. In 

 Nereis at the fifty-eight-cell stage there are thirty-two cells in the 

 first quartet, and a similar number in DinopJiilvs at the sixty-five-cell 

 stage. 



Soon after the last division the cells la^-^-^-^-ld^-^-^' divide again in 

 a slightly dexiotropic direction (fig. 28). This time the distal cells, 

 2^i.i.2.2.2_j^i.i.2.2.2^ are the smaller and lie opposite the cells from whicli 



