66 



COMPARATIVE PHYSIOLOGY. 



stage " is the obvious one of apparent quiescence — cessation of 

 all kinds of movement. Then ensues rapidly and in succession 

 the following series of transformations : The nucleolus divides, 

 later the nucleus, into two parts. These new nuclei then wan- 

 der away from each other in opposite directions, and, losing 

 their character as nuclei and nucleoli, are replaced by asters 

 (polar stars), which seem to arise in the protoplasm of the body 



Fig. 64.— Early stages of segmentation of a moUusk, Elysia mridis (drawn from the 

 living egg). A, oBspenn in state of rest after the extrusion of the polar cells; B, 

 the nucleolus alone has divided; C, the naclens Is dividing; D, the nuclens, as 

 such, has disappeared, first segmentation farrow appears; E, later stage; F, 

 oosperm divided into two distinct segmentation spheres, the clear nnclear space 

 in the center of the aster of jgrannles is growing larger; G, resting stage of ap- 

 preseed two spheres; H, I, similar stages in the prodnction of fonr spheres; K, 

 formation of eight-celled stage. (Haddon.) 



of the cell, and which are in close juxtaposition at first, but later 

 separate, the oosperm becoming amoeboid in one region at least. 

 A groove, which gradually deepens, appears on the surface, and 

 finally divides the cell into two halves, which at once become 

 flattened against each other. The nucleus may again be recog- 

 nized in the center of each polar star, while a new nucleolus 

 also reappears within the nucleus, when again a brief period of 

 rest ensues. In the division and reformation of the nucleus, 

 when most complicated {karyoJcineeis), the changes may be gen- 



