242 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Although for convenience I shall continue to use the term equatorial 
band, it is clear that the cells composing it are no longer strictly equato- 
rial in position, but now lie on the flattened dorsal surface (Fig. 62). 
This change of position has come about in consequence of the more 
rapid cell division in the ventral hemisphere. How considerable the dif- 
ference in rate of division has been between the cells of the two hemi- 
spheres, one readily appreciates if he stops to consider that the cells of 
the ventral hemisphere now number fifty-four, whereas those of the 
dorsal hemisphere number only twenty-two. 
In the dorsal hemisphere (Fig. 62) the divisions foreshadowed by 
spindles at the 64-cell stage (Fig. 60) have taken place, but no new 
ones are approaching. The number of cells in this hemisphere is now 
twenty-two; twelve of them (the chorda and mesoderm cells, cf. descrip- 
tion of Fig. 62 in the explanation of Plate X.) are in the seventh gen- 
eration, and ten (the endoderm fundament), in the sixth generation, no 
divisions having occurred in the last named group of cells since the 32- 
cell stage. Of the ten chorda cells, eight derived from the anterior quad- 
rants are arranged in a crescent-shaped band capping the anterior end of 
the dorsal hemisphere ; they are a’, a", a1, a™™, and the correspond- 
ing cells in quadrant B. They form the anterior chorda fundament. 
The other two chorda cells, which are derived from the posterior quad- 
rants, are d" and c, "They form the posterior chorda fundament, 
and are at present separated from the anterior chorda cells by two cells 
of the equatorial band, viz. 47:5 and BS. 
The sister cells of d"! and ec", viz. d? and c'??, are the sole con- 
tribution of the dorsal hemisphere to the mesoderm of the larva, for the 
greater part of the mesoderm is, as we shall see, derived from the equa- 
torial band. 
Among the endoderm cells it is noticeable that d 95 and its mate c*5 
have been shoved forward out of their own quadrants to a position 
beside the endoderm cells derived from the anterior quadrants. 
(e) Summary on Later Cleavage Stages, 
1. In the cleaving óvum one can recognizo, in passing from the ani- 
mal to the vegetative pole successive zones, in each of which cleavage 
takes place less rapidly than in the preceding. At the 64-cell stage 
(Plate X. Figs. 59, 60) there are three such zonos: first, a group of 
thirty-two cells encircling the animal pole, all of them in the eighth 
generation ; second, an equatorial zone of sixteen cells, all in the seventh 
generation ; third, a group of sixteen cells encircling the vegetative pole, 
