24 KARYOKINESIS. 



the egg sphere as long as the sperm nucleus is smaller than that of the egg, lends 

 further weight to this suggestion. 



The earliest stage in the formation of the sperm aster which I have seen is 

 shoAvn in figs. 39 and 40. I have examined thousands of eggs of earlier stages, but 

 have failed to find a sperm aster in any of them. The aster when first seen is a 

 radiating figure in the cytoplasm, with several dark granules at its center. The 

 number, position and size of these granules is not constant, and in later stages they 

 greatly increase in number and stain less darkly than at first ; there can be little 

 doubt that they are identical with the granules derived from the middle piece. The 

 sperm aster with the granules at its center ultimately becomes more rounded in 

 outline and forms a large sphere from which radiating fibers proceed in all direc- 

 tions. This sphere exactly resembles the sphere in contact with the egg nucleus, 

 fig. 41. 



From the time of their first appearance each of these spheres lies close to its 

 own nucleus, and they do not wander from these relative positions so that there is 

 no possibility of confusing or mistaking them. During the approach of the sperm 

 nucleus and aster to those of the egg, one or two small accessory asters appear in 

 the egg, usually at some distance from the sperm and egg nuclei (figs. 42 and 43) ; 

 these resemble the minute asters described by Mead ('98), and Lillie ('98) as 

 " accessory asters." They contain no centrosomes or large granules, and their 

 origin at a distance from the egg and sperm asters shows that they are independent 

 of either of these. These accessory asters are present for a brief period only and 

 then completely disappear. 



At no stage in their development do the egg and sperm spheres show the com- 

 pact and densely staining qualities which the spheres show throughout the cleavage 

 stages ; this added to the fact that there is a less perfect separation of cytoplasm 

 and yolk during the fertilization than in the cleavage makes the study of these 

 structures difficult, and this is especially true in the stages just before and after 

 the fusion of the spheres. While designating these structures "spheres," both 

 because of their form and also because of the derivation of the egg sphere from 

 the sphere left in the egg at the close of the second maturation, I would not be 

 understood as positively homologising them with the "outer sphere" or "cortical 

 zone" of authors. 



4. Approach of Germ Nuclei and Spheres. — The egg nucleus and sphere 

 remain at the upper pole, immediately beneath the polar bodies, and do not move 

 from this position. The sperm nucleus and sphere move toward those of the egg in 

 a path which is at first directed toward the center of the egg (" Entrance path," 

 Roux), and then toward the egg nucleus (" Copulation path"). If the sperm enters 

 near the lower pole, the course of the sperm elements is nearty straight through 

 the egg from the lower to the upper pole ; if it enters at any other point than the 

 vegetal pole, the path is a curved one, the "entrance path" curving more or less 

 sharply into the " copulation path," depending upon the distance of the point of 

 entrance from the vegetal pole. In all cases the sperm elements approach those of 



