1904.] NATURAL SCIENCES OF PHILADELPHIA. 339 
sion walls of the first quartet, but they do not appear to become so 
flattened as their predecessors (figs, 13, 14): 
The Trochoblasts.—Before the macromeres again divide the first 
quartet is seen to be in process of cleavage. There result eight cells 
of nearly equal size, the more peripheral being slightly smaller than 
those at the apical pole. The spindles which precede division are 
lxotropically directed, and the lower cells are pushed downward and 
outward between the second quartet cells and just above the macro- 
meres (figs. 15, 16). These “primary trochoblasts” or “turret cells” 
do not again divide until about sixty cells are present (Pl. XXV, 
figs. 33, 38), when they have become considerably flattened and lie 
between the arms of the forming ectoblastic cross. The fate of these 
very characteristic cells will be discussed later. 
Third Quartet and First Division of Second Quartet.—The first 
division of the second quartet and the third division of the macro- 
meres occur simultaneously. Each second quartet cell forms two 
of equal size by a distinctly dexiotropic cleavage, the spindles being 
from the first inclined in that direction. As may be seen in figs. 17 
and 18, these cells do not all divide at exactly the same time, and this 
lack of regularity is also characteristic of the macromeres. By this 
division of the second quartet the eight cells of the first are pushed back- 
ward dexiotropically so that, in relation to the macromeres, they occupy 
the same place as when given off. The division of the macromeres 
results in the four cells of the third quartet. They arise in a dexiotropic 
manner and are equal in size to the four cells left at the lower pole. 
From this stage on these latter are ‘“‘macromeres” in name only, being 
equalled in size by the third quartet and but slightly larger than the 
eight derivatives of the second. Nor, indeed, do the macromeres 
appear at this stage to contain much more yolk than the micromeres, 
At a later period they are easily discernible from the micromeres by 
their clear yellow appearance, but as the latter divide much more rap- 
idly and by growth distribute the yolk which they contain over a 
larger area, while much of it is doubtless absorbed, the preponderance 
of this material in the individual cells of the endoderm and the larger 
cells of the mesoderm as well is easily explained. As has been men- 
tioned before, in the larva the amount of yolk in ectodermal struc- 
tures is quite considerable, showing its wide and universal distribu- 
tion throughout the entire organism. 
The twenty-four-cell stage has thus been reached and as yet the egg 
