RELATION BETWEEN AMITOSIS AND MITOSIS. 1 43 



functional spermatozoa arise from the "spermatid" nuclei, which 

 themselves arise by fragmentation of the spermatocyte-nuclei, 1 

 we shall be forced to the conclusion that fertilization is possible 

 without the typical process of maturation. 



III. Cleavage. 



The relations of the two pronuclei at the time of the first cleav- 

 age varies considerably in different eggs. In some cases the 

 chromosomes form (Fig. 12, PL III.) and the membrane disappears 

 while the pronuclei are still more or less widely separated. In 

 such cases the chromosomes appear in two distinct groups on the 

 two sides of the spindle (Figs. 15 and 16, PI. IV.). In other 

 cases the pronuclei approach more closely (Fig. 13, PL III.) or even 

 undergo more or less complete fusion (Fig. 14, PL III.) before the 

 disappearance of the membrane. In such cases the chromosomes 

 may or may not appear in two groups according to the complete- 

 ness of the fusion. In one case the chromosomes of the first 

 cleavage were observed in two distinct groups within a single 

 nuclear membrane (Fig. 17, PL IV.) and in many cases no trace 

 of the paternal and maternal groups was visible (Figs. 18, 19, 20, 

 PL IV.). 



The position of the first cleavage-spindle varies greatly as is 

 evident from Figs. 15, 16, 18, 19 and 20 (PL IV.). Since nu- 

 clear division proceeds much more rapidly than cytoplasmic divi- 

 sion during early cleavage, and since the polar bodies soon de- 

 generate or are absorbed, it has not been possible to determine 

 whether the plane of the first cleavage is uniform in position, i. e., 

 whether the first cleavage -spindle finally attains a definite typical 

 orientation in the egg. From my observations on cleavage stages 

 I am somewhat inclined to doubt that this is the case, although 

 my data are not conclusive. 



There can be no doubt that the first cleavage is usually mitotic 

 but occasionally conditions are found which might readily be 

 regarded as cases of amitosis. Frequently eggs containing only 

 a large single nucleus which is apparently giving rise amitotically 

 to a smaller nucleus are found. None of these cases are figured 

 since it is by no means certain that they are normal phenomena. 



1 Child, C. M., Biol. Bull., XII., 4, 1907. 



