232 MEAD. [Vol. XIII. 



brane, and is perfectly opaque. The cleavage nucleus is slightly- 

 eccentric, lying nearer the polar globules, and is surrounded 

 by a thick layer of protoplasm, while the yolk is mostly segre- 

 gated at the vegetative pole. For descriptive purposes the 

 position of the polar bodies indicates approximately the ani- 

 mal pole of the &gg and the center of the anterior he^nisphere ; 

 the point 180° from this is the vegetative pole, the center of 

 the posterior hemisphere ; the line joining the two points is the 

 vertical axis of the egg. 



When the first cleavage spindle is formed, the e.gg is slightly 

 oval, and the spindle always parallel with the long axis, on the 

 same level as the segmentation nucleus, and nearer one end of 

 the egg (Fig. i, PI. X). At this time the tgg is completely 

 oriented with reference to the future cleavage furrows. The 

 rate of cleavage is influenced directly by the temperature ; in 

 July the first polar globule appears in less than half an hour 

 after fertilization, and the first cleavage furrow about thirty 

 minutes later. 



The first furrow is meridional (parallel with vertical axis), 

 and appears all round the egg at about the same time, though 

 it sinks in somewhat more rapidly on the anterior hemisphere. 

 It divides the egg into two blastomeres of unequal size, A-B 

 and C-D (Fig. 2), and in a few minutes these divide nearly 

 simultaneously, though the larger (C-D) is sometimes a little 

 in advance. The direction of the cleavage is always left 

 oblique ; the karyokinetic spindles, even when in the equatorial- 

 plate stage, are inclined to the plane of the equator, and indi- 

 cate the direction of the cleavage (Fig. 3). Consequently the 

 cells B and D in the 4-cell stage lie somewhat lower (nearer 

 the vegetative pole) than A and C, and meet in a " cross- 

 furrow" (Figs. 4, 5); but the "precocious formation" of the 

 cross-furrow found in some eggs, for example, in Crepidiila and 

 Nereis, is not apparent in Amphitrite. In the cleavage of the 

 two blastomeres, as in the first cleavage, the furrows sink in 

 more rapidly on the anterior hemisphere than on the yolk- 

 laden posterior hemisphere (Fig. 4). 



The cell D of the 4-cell stage (Figs. 4-6) is considerably 

 larger than the other cells, and its descendants play a conspicu- 



