MATURATION OF THE EGG, AND PROCESS OF FERTILISATION. 31 



of the nucleus which one encounters in the animal and vegetable 

 kingdoms in stages preparatory to cell-division. 



The nuclear spindle, the more precise structure of which will be 

 described later, in discussing the process of cleavage, pursues still 

 further the direction already taken by the germinative vesicle, until 

 it touches with its apex the surface of the yolk, where it assumes a 

 position with its long axis in the direction of a radius (fig. 13 / sp). 

 A genuine process of cell-division soon takes place here, which is to 

 be distinguished from the ordinary cell-division only by this, that 

 the two products of the division ,are of very unequal size. To be 



Fi^. 12. — Portions of eggs of Asterias glacialis. They show the degeneration of the germinative 

 vesicle. 



In figure A it begins to shrivel, in that a protuberance of protoplasm (x), with a radial stinicture 

 inside of it, penetrates into its interior, and dissolves the membrane at that point. Thp 

 germinative dot (,lcf) is still visible, but separated into two substances, nuclein (nu) and 

 paranuclein (pn). 



In figure J3 the germinative vesicle (kb) is entirely shrivelled, its membrane is dissolved, and 

 only small fragments of the germinative dot (kf) remain. In the region of the protoplasmic 

 protuberance of figure A there is a nuclear spindle C^) in process of formation. 



more exact, therefore, we have to do here with a cell-budding~ At 

 the place where the nuclear spindle touches the surface with one of 

 its extremities the yolk arches up into a small knob, into which 

 half of the spindle itself advances (fig. 13 //). The knob thereupon 

 becomes constricted at its base, and with the half of the spindle — 

 from which subsequently a vesicular nucleus is again formed — is 

 detached from the yolk as a very small cell (fig. 13 III r¥). Here- 

 upon exactly the same process is repeated, after the half of the 

 spindle which remains in the egg, without having previously entered 

 into the vesicular quiescent stage of the nucleus, has restored itself 

 to a complete spindle (fig. 13 IV). 



There now lie close together on the surface of the yolk two 

 spherules, which consist of protoplasm and nucleus, and therefore 

 have the value of small cells (fig. 13 F rlc^, rP), and which are 

 often to be identified in an unaltered condition, even after the 

 egg has been divided into a number of celLs. They were already 



