240 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



the nervous system of the larva. (See Fig. 60, A''-'^, A'''^, B'-\ and B''-^.) 

 The two remaining mitotic cells of the equatorial band are situated later- 

 ally one n each of the posterior quadrants (Figs. 59 and GO, D'^'^, C'^). 



The ten remaining cells of the equatorial band all contain resting 

 nuclei. Eight of these cells are grouped at the esti'eme posterior end of 

 the equatorial band in a region where, from the 16-cell stage on, we have 

 found cleavage to be more tardy than in any other part of the ventral 

 hemisphere. These eight cells are D''-^, D'-^, D''', Zf-^, and the corre- 

 sponding cells in quadrant C. (Figs. 59 and 60. Compare Fig. 57.) The 

 two remaining cells of the equatorial band which still show no signs of 

 division are A'''^ and £''^ (Fig. 60), situated about midway between the 

 anterior and posterior ends of the embryo. 



Of the sixteen cells comprising the dorsal hemisphere (Fig. 60), six, 

 which lie in contact with the equatorial band («®-^, a^-'', d^-^, with their 

 mates in quadrants B and C), are in mitosis. Four of them, the most 

 anterior of the cells of the dorsal hemisphere, lie in a transverse row 

 across the dorsal surface of the embryo (Fig. 60, a®', o®-^, b^-^, and b^-''). 

 They will ultimately form the greater portion of the chorda. We will 

 call them the anterior chorda fundament. The two other mitotic cells 

 of the dorsal hemisphere ai"c d^'^ and c®-®, in the posterior half of the 

 embryo (Fig. 60). The spindles in these cells are directed obliquely 

 forward, upward, and outward, so that, taking into considei'ation the 

 superficial position of the nuclei of the dorsal hemisphere, we may pre- 

 dict that the coming division will result in cutting off in each case a 

 smaller more superficial cell from a larger cell extending deeper ; the 

 small cell will also lie anterior and lateral to its sister cell. The unequal 

 divisions in these two cases will separate cells of unlike fate ; the two 

 smaller cells will constitute the posterior chorda fundament, the two 

 larger ones will form mesoderm. 



The ten remaining cells of the dorsal hemisphere (Fig. 60, a^-^, a^-^, 

 d^-^, d^'^, d^-^, and the corresponding cells in quadi-ants JJ and C) show as 

 yet no signs of division. They are grouped about the vegetative pole of 

 the egg, the point of origin of the polar cells, and will form the whole 

 of the definitive endoderm of the lari^a, and nothing else. 



(d) 76-ceU Stage. 



Upon the completion of division in the twelve mitotic cells of the 

 embryo represented in Plate X. Figs. 59 and GO, a stage of seventy-six 

 cells would be reached. An embryo in this stage is shown in Plate X. 

 Fig's. 61 and 62. It contains in the 



