THE RELATION BETWEEN AMITOSIS AND MITOSIS. 199 



undergo fragmentation again. At any rate such cases are rare 

 and not of fundamental importance. In one case a mitotic spindle 

 was observed in a cytophore whose nuclei had apparently arisen 

 by fragmentation since two nucleoli were present and the nuclei 

 were not at the periphery (Fig. 41, PI. XIV.). This would ap- 

 pear to be a case where a nucleus resulting from fragmentation 

 divides mitotically afterward : it might, however, be a case where 

 a spermatocyte undergoing mitosis had fused with a group under- 

 going fragmentation : if that were the case the size of the spindle 

 would seem to identify it as the second spermatocytic division. 

 But whatever its interpretation, this, too, is clearly an exceptional 

 case. It is probable that a large proportion of the nuclear sub- 

 stance of the spermatocyte passes into the cytoplasm at the time 

 of fragmentation. It may be that a kind of reduction is accom- 

 plished in this manner. 



The relative frequency of fragmentation and mitosis seems to 

 vary in different chains, proglottids and testes. In some chains 

 the spermatogenetic mitoses were rarely seen except in the older 

 proglottids, yet spermatozoa were produced in the younger pro- 

 glottids as abundantly apparently as elsewhere. In some other 

 cases mitosis is more frequent. From examination of the testes 

 one gains the impression that the mitoses are not in any case 

 sufficiently numerous to account for the large number of sperma- 

 tids and spermatozoa formed. I regard it as at least probable 

 that spermatozoa are produced from the "spermatid" nuclei 

 which arise by fragmentation, as well as even those which arise 

 by mitosis. As will appear in a later section, some cells undergo 

 degeneration in almost all or all testes and it is of course im- 

 possible to prove that these particular spermatid nuclei which arise 

 by fragmentation do not undergo degeneration. Still, developing 

 spermatozoa have been found on cytophores containing the old 

 spermatocyte nucleoli. 



VI. The Formation of the Spermatozoon from the Spermatid. 



In consequence of the difficulty of observation of details in 

 these exceedingly minute structures, which is farther increased 

 by the massing together of the spermatids in cytophores and the 

 condensation of these in later stages, it has not been possible 

 to reach positive conclusions on all points. It can scarcely be 



