90 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



to the first cleavage plane, and passes through the animal pole of the egg 

 (Figs. 29, 30). 



The spindle in the yolk-cell cd- is eccentric in position, lying nearer 

 the animal pole of the egg, and near the centre of the protoplasmic 

 mass; it is nearly perpendicular to the chief axis (Fig. 29). As cleavage 

 progresses the spindle becomes inclined so that one end dips into the 

 yolk-mass, which lies at the vegetative pole of the yolk-cell (Figs. 31 and 

 99). From the point of view of a miniature observer occupying the 

 chief axis of the ovum with his head directed toward the animal pole, the 

 left end of the spindle is the one that is nearer the animal pole, that is, 

 the spindle is laeotropically oblique. Usually the spindle makes an 

 angle of about 30° or 40° with tiie chief axis. 



The yolk-cell cd'^ cleaves unequally, and the cleavage plane may be 

 considered a modified meridional one. The cleavage planes of the "pro- 

 toplasmic " cell ab^ and of the yolk-cell meet in a line which passes 

 through the animal pole, but does not coincide with tlie chief axis ; it 

 makes with this axis an angle of about 45°. To our imaginary observer 

 the resulting smaller cell (c^) lies to the left of and above the larger or 

 yolk-cell d^ (Fig. 31), and also this cell lies above the anterior cell b^. 

 The cell c* is the second micromere which is separated from the yolk. 



At tlie close of the second cleavage a general tendency towards a Ino- 

 tropic arrangement of the cells is noticed (Figs. 32-34, 100-102). This 

 arrangement in the case of the posterior cells (c", d^) is apparently the 

 result of the oblique position of the spindle in the yolk-cell cd'^. When- 

 ever the anterior cell ab^ (first micromere) divides in advance of the yolk- 

 cell cd', there is no suggestion of a lipotropic arrangement either in its 

 spindle or in the position of the resulting cells (a^, J/, Fig. 99) ; but 

 after cleavage of the yolk-cell, the right anterior cell i* is depressed by 

 the higher lying cell c'. This change can be seen in the living ovum as 

 the cleavage of the yolk-cell cd^ progresses. 



Soon after the completion of the second cleavage the four cells tend 

 to become rounded, and adjustments of position occur. Figures 32-35 

 and 102, 103 represent the arrangements which are usually seen, and 

 in all of them a definite plan can be recognized. The axis of tlie future 

 eiul)ryo can now be described as passing through the nuclei of the an- 

 terior cell, i', and of the yolk-cell, rf* (Fig. 31). The anterior cell, b^, 

 always comes to lie nearer tlie vegetative pole than the cells a* and c', 

 and it is usually more or less covered on the animal side by one or both 

 of these cells (Figs. 34, 35). After examining the eight-ccU stage, in 

 which the bilateral symmetry is distinctly marked, it will be seen that 



