REPRODUCTION. 65 
the nucleus, into two parts. These new nuclei then wander 
away from each other in opposite directions, and, losing their 
Fia. 68.—Early stages of segmentation of a mollusk, Elysia viridis (drawn from the living 
eg). A, odsperm in state of rest after the extrusion of the polar cells eo the nucleolus 
alone has divided ; C, the nucleus is dividing ; D, the nucleus, as such, has femeel pret 
poqtieniation auneren aie Gear vubleur'snase i the’ carter Gf the eater OF prams ia 
growing larger; G, resting stage of appressed two spheres; H, I, similar stages in the 
production of four spheres ; K, formation of eight-celled stage. (Haddon.) 
character as nuclei and nucleoli, are replaced by asters (polar 
stars), which seem to arise in the protoplasm of the body of 
the cell, and which are in close juxtaposition at first, but later 
separate, the odsperm becoming amoeboid in one region at 
least. A groove, which gradually deepens, appears on the sur- 
face, and finally divides the cell into two halves, which at once 
become flattened against each other. The nucleus may again 
be recognized in the center of each polar star, while a new nu- 
cleolus also reappears within the nucleus, when again a brief 
period of rest ensues. In the division and reformation of the 
nucleus, when most complicated (karyokinesis), the changes 
may be generalized as consisting of division and segregation, 
followed by aggregation. 
The subdivision (segmentation) of the cell, after the quies- 
cence referred to, again commences, but in a plane at right 
angles to the first, from which four spheres result, again to be 
followed by the resting stage. The process continues in the 
same way, so that there is a progressive increase in the num- 
ber of segments, at least up to the point when a large number 
5 
