332 PROCEEDINGS OF THE ACADEMY OF [April, 
Before segmentation, the nucleus lies but slightly above the center 
of the egg, having moved downward with its surrounding mass of 
granular protoplasm. An extremely thin and easily ruptured vitelline 
membrane surrounds the egg, and on account of the delicacy of this 
membrane no micropyle is present. Usually 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. rom 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 Crepidula (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 
