240 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
the nervous system of the larva. (See Fig. 60, A’, A75, B™4, and B"*,) 
The two remaining mitotic cells of the equatorial band are situated later- 
ally one in each of the posterior quadrants (Figs. 59 and 60, D"*, 07%), 
The ten remaining cells of the equatorial band all contain resting 
nuclei. Kight of these cells are grouped at the extreme 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"5, DT, Di", D'S, 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 B"** (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**, a9", d°°, with their 
mates in quadrants Band 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*", a*5, 595, and DA 
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 are d** and c*5, 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 consideration 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 des 
des, d%, d**, and the corresponding cells in quadrants B and C) show as 
yet no signs of division. They are grouped about the vegetativo pole of 
the egg, the point of origin of the polar cells, and will ‚form the whole 
of the definitive endoderm of the larva, and nothing else. 
(d) 76-cell Stage. 
Upon the completion of division in the twelve mitotic cells of the 
embryo represented in Plate X. Figs. 59 and 60, a Stage of seventy-six 
cells would be reached. An embryo in this stage is shown in Plate X. 
Figs. 61 and 62. It contains in the 
