1904.] NATURAL SCIENCES OF PHILADELPHIA. 335 



sion walls of the first quartet, but they do not appear to become so 

 flattened as their predecessors (figs. 13, 14). 



The Trochohlasts. — 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 

 Iseotropically 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 (PI. XXV, 

 figs. 33, 38), when they have become considerably flattened and he 

 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 shghtly 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, showdng its wide and universal distribu- 

 tion throughout the entire organism. 



The twenty-four-cell stage has thus been reached and as yet the egg 



