1890.] [Ryder. 



plasm, the process of karyokinesis or movement of the fragments of 

 chromatin is finally restricted to such an extent, from the want of a cyto- 

 plasmic field, that nuclear movement is at last rendered impossible. 



Much as the lengthening of the spermatozoon resembles a diastolic 

 phase of karyokinesis, there is no evidence that the elongation of the 

 male element preparatory to being set free, can be identified in any case 

 with such a final karyokinetic diastolic phase. If this were so it might be 

 supposed that the momentum of karyokinesis, in this case, had reached a 

 potential state or condition of tension ready to exhibit itself as segmenta- 

 tion, as soon as there was a large enough cytoplasmic field, as in the cyto- 

 plasm of the egg, in which the opposite condition of systole could occur, 

 and thus bring back the nucleus to a condition of equilibrium. 



While the foregoing conclusion cannot be assumed, it may be assumed 

 that the male cells, in undergoing their rapid multiplication, do acquire a 

 certain karyokinetic momentum predisposing them to set up segmentation 

 in other functionless plasmic bodies ova which they may enter. Yet, 

 as we have seen, karyokinesis is not always an accompaniment of sper- 

 matogenesis, a condition which may arise, as supposed above, from the 

 gradual diminution of the cytoplasmic field. 



The method of evolution of spermatozoa is not uniform in all cases. In 

 Ostrea edulis there are rarely individuals in which spermatogenesis ap- 

 proximates that of 0. mrginica. Then, rarely, intermediate forms occur 

 between this and the normal form, where large masses of chromatin are 

 formed by direct elaboration from the nuclei of elements which cannot be 

 regarded as other than ova. In the latter case the metabolism which 

 leads to the development of spermatozoa is clearly carried a step further 

 than in ovogenesis, because the huge masses of chromatin imbedded in 

 the ova from which the nuclei of the spermatozoa are formed are very 

 certainly developed after the stage is reached which answers to that of the 

 mature ovum. The male condition is reached therefore in this last case after 

 the female, and is an outgrowth of the latter consequent upon the develop- 

 ment of large masses of chromatin in the egg and its direct fragmentation 

 into the nuclei of spermatozoa. In those cases where the spermatozoa are 

 developed more directly from smaller cells which never reach the dimen- 

 sions of ova, we have a totally different case, and one which indicates a 

 protandrous tendency. The other case where the male condition depends 

 upon the previous development of a fully differentiated female state of 

 the germ-cells obviously corresponds to a protogynous condition. 



The formation of chromatin in the last case does not proceed as a result 

 of metabolism and growth following a rapid series of karyokineses alter- 

 nating with periods of rest, but follows the formation of a female nucleus 

 in which a rapid endogenous formation of chromatin first occurs, followed, 

 as it increases in bulk, by the extrusion of the chromatin from the parent 

 nucleus into the surrounding cytoplasm, where it breaks up into small 

 masses which are later separated in large groups as the nuclear basis of 

 large coherent clumps of spermatozoa. 



