238 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



from the anterior and posterior ends respectively, rather than from the 

 dorsal and ventral surfaces, as in most of the other stages figured. This 

 stage (48-cell) is made up as follows : — 



Ventral hemisphere, 32 cells in the seventh generation. 

 Dorsal hemisphere, 16 cells in the sixth generation. 



48 



It will be observed that the cells of the ventral hemisphere, though all 

 in the seventh generation, are not all equally advanced in their prepara- 

 tions for division, which evidently is again about to set in. For while 

 the cells occupying the centre of the ventral hemisphere, or, in other 

 words, lying nearest to the animal pole of the egg, are about to pass 

 into the next generation, the cells occupying the margin of the ventral 

 hemisphere, and more remote from the animal pole, contain nuclei en- 

 tirely quiescent, like those seen in the cells of the dorsal hemisphere. 

 This is contrary to the statement made in my preliminary notice ('94), 

 in which I said that at this division those cells of the ectodermal hemi- 

 sphere which were marginal and in contact with cells of the endodermal 

 hemisphere were first to divide. This erroneous statement arose from 

 the wrong interpretation given to Figures 11 and 12 ('94 Plate II.) in 

 describing the cell lineage of that stage, a matter to which attention has 

 already been directed. 



In the embryo shown in Plate X. Figs. 57 and 58 (48-cell stage) one 

 may readily distinguish three regions, each composed of sixteen cells. 

 The first region is the dorsal hemisphere, with its sixteen cells all in the 

 sixth generation (a*-^-«'' ^ d^^-d^-^, and the corresponding cells in quad- 

 rants B and C). These cells are destined to form the endoderm of the 

 larva, the chorda, and a portion of the mesoderm. The second group of 

 sixteen cells occupies the centre of the ventral hemisphere {A''-^, A'-^, A'^, 

 A''-^, A''-'', W ^, D"^-', and D''-^, with the corresponding cells in quadrants 

 B and C). They are in the seventh generation, but already contain 

 spindles, showing that they are soon to pass into the eighth generation. 

 This group of cells will form the ectoderm of the larva. The remain- 

 ing sixteen cells of this embryo, also belonging to the ventral hemi- 

 sphere, form the third group {A-\ ^'•^ ^'-S and D''^-D''\ with the 

 corresponding cells in quadrants B and C). They too are in the seventh 

 generation, but their nuclei are quiescent, showing that these cells will 

 be later in dividing than the other cells of the ventral hemisphere. 

 They are arranged in an equatorial band between the other two groups 



