CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 239 



of cells. This band is inteiTupted at only one point on each side of the 

 embryo, where a single cell (A^-^, B''-^) of the ectodermal group reaches 

 lip into contact with the cells of the dorsal hemisphere. From the equa- 

 torial band just described are derived chiefly nerve cells and mesoderm 

 cells. 



(c) 6Jf.-cell Stage. 



The completion of the divisions foreshadowed by spindles in the ecto- 

 dermal group of cells of the stage last discussed (Plate X. Figs. 57 

 and 58) doubles the number of cells in that group, and brings the num- 

 ber in the entire embryo up to sixty-foui', distributed as follows. 



Ventral hemisphere (designated by the letters A, B, O, J)) : — 

 32 cells in the 8th generation = the ectodermal group. 

 16 " 7th " := the equatorial band. 



48 

 Dorsal hemisphere (designated by a, by c, d) : — 

 16 cells in the 6th generation. 



64 



Such a stage is shown in Plate X. Figs. 59 and 60, the former 

 representing a ventral and the latter a dorsal view. The egg has again 

 assumed the flattened form which it had at the 32-cell stage. 



Examining first the ventral surface (Fig. 59), we see that the divisions 

 foreshadowed in the 48-cell stage (Figs. 57 and 58) have in every in- 

 stance occurred in a direction perpendicular to that of the spindle in 

 the mother cell, though a slight displacement is in some cases appear- 

 ing among the daughter cells, on account of the mitoses arising in the 

 equatorial band. The cells of the ectodermal group, on account of their 

 recent division, now number thirty-two, as many as are found in both 

 the other groups put together. They are in the eighth generation, one 

 generation in advance of the cells of the equatorial band, and two gen- 

 erations in advance of tlie cells of the dorsal hemisphere. They are 

 ^s.i_js.G^ .|8.9^ J8.W ^8,i:i^ ^|8.i4^ ^^^^i 2)S.i_j)S.c,^ togothcr with the Correspond- 

 ing cells in quadrants B and C. 



The equatoi'ial band is, as at the last stage, composed of sixteen cells 

 all in the seventh generation, l)nt six of them (three on each side of the 

 median plane, Fig. 60, A^'^ A''-^, and I^'*) now show signs of approaching 

 division. Four of these mitotic cells form the anterior segment of the 

 equatorial band, and are destined to produce a considerable portion of 



