242 BULLETIN : MUSEUM OF COMPAKATIVE ZOOLOGY. 



Although for couveuience I shall coutiuue 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- 

 sphei"es, one readily appreciates if he stops to consider that the cells of 

 the ventral hemisphere now number tifty-four, whereas those of the 

 dorsal hemisphere number only twenty-two. 



In the dorsal hemispliere (Fig. 02) 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^-^°, a*'^^, a''^*, and the correspond- 

 ing cells in quadrant £. 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. A'-^ and B^-^. 



The sister cells of d''-^^ and c''^^, 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 tlie endoderm cells it is noticeable that d^-^ and its mate c^-^ 

 have been shoved forward out of their own quadrants to a position 

 beside the endoderm cells derived from the anterior quadrants. 



(e) Siimniayy on Later Cleavage Stages. 



1. In the cleaving ovum one can recognize, 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 zones : 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. 



