332 PROCEEDINGS OF THE ACADEMY OF [April, 



r Before segmentation the nucleus lies but slightly above the center 

 of the egg, having moved doAvnward with its surrounding mass of 

 granular protoplasm. An extremely thin and easily ruptured vitelline 

 membrane surrounds the egg, and on account of the dehcacy of this 

 membrane no micropyle is present. Usuallj' one but often two or 

 three eggs lie together within a roomy egg capsule, containing also a 

 fluid substance which does not coagulate in reagents. In unstained 

 fixed material, and also doubtless in the living state, the eggs are quite 

 opaque from the yolk which they contain. 



First Cleavage. 



The first cleavage is initiated by nuclear rupture and increased evi- 

 dence of stellar radiation. With the formation and elongation of the 

 spindle the surrounding yolk spherules give place to the more proto- 

 plasmic constituents of the cell which form the immediate nuclear 

 environs. The spindle as it elongates moves somewhat farther down- 

 ward in the egg and lies but slightly above the equatorial plane. In 

 length it measures about half the diameter of the egg. From the first 

 constriction is almost equally marked all around the egg, though 

 slightly greater at the animal pole. After the chromosomes have 

 separated and are moving toward the opposite ends of the spindle, one 

 end appears somewhat higher than the other (fig. 5), a position which 

 would indicate a spiral trend of cleavage; but this is not evident in 

 the telophase and completed division, for in the two-cell stage the 

 nuclei lie directly opposite each other. 



As in the usual history of cleaving eggs, the resulting blastomeres 

 are at first much rounded, but as their nuclei form they become closely 

 pressed together, forming a flattened contact surface between which 

 no cleavage cavity exists (fig. 6). The nuclei, together with their 

 surrounding cytoplasm, again approach the upper surface of the egg 

 and lie at rest just beneath the surface on opposite sides of the polar 

 bodies. There is no evidence in their position to indicate a "virtual" 

 rotation before the next cleavage, as is the case in Crepidida (Conklin, 

 1897). The daughter nuclei of the first cleavage becomes much dilated, 

 containing several nucleoli suspended in the chromatin network and 

 surrounded by clear nuclear fluid. 



The two blastomeres thus formed are equal or so nearly equal in 

 size that they present to the observer no mark of distinction, and it 

 can only be conjectured which will form the anterior and which the 

 posterior region of the larva. Indeed, not until the appearance of 

 the mesentodermal cell at the close of the twenty-four-cell stage can 



