Origin of Sexuality 279 



direction. The male pronucleus, at first huddled up at one 

 pole of this nuclear spindle, also breaks up into granules of 

 chromatin, wliich mingle with those of the female pronucleus, 

 and cause the spindle to increase in size until it finally stretches 

 through the entire oocyst. One pole of the spindle marks 

 the point of entry of the microgamete, where its disappointed 

 companions are still to be seen outside the oocyst. The pe- 

 culiar spindle-like arrangement of the nucleus in the zygote 

 has nothing to do with nuclear division, but is simply a means 

 of mixing intimately the chromatin derived from two different 

 sources, and when this is effected the spindle contracts and 

 rounds itself off, the final result being a spherical nucleus, 

 the fusion-product of the two pronuclei." 



The details for the Hsemosporidia in their main outlines 

 resemble the last. 



The large series of the Myxosporidia seems to be a group 

 of organisms w^hich, by parasitic degradation, has lost the 

 sexual phase during their life-cycle, or this is reduced to simple 

 conjugation between small and similar gametes. 



In the important group of Hsemoflagellata, mature trypan- 

 osomes become transformed mto an egg in one case and a 

 cluster usually of eight sperms in another. In both instances, 

 a certain amount of chromatin material undergoes disintegra- 

 tion, and is absorbed or extruded. The remainder, during 

 differentiation of the egg-cell, becomes a large female nucleus 

 that, after fertilization, produces the trophonuclear and kineto- 

 nuclear constituents of the adult. The remaining chromatin 

 substance of the male cyst produces eight double nuclei, made 

 up each of a tropho- and kinetonuclear constituent. Round 

 these two constituents a motile sperm mass forms, and, on 

 escape, one fertilizes a large egg. 



The sexual condition met with in Infusoria is in the nature 

 of a conjugation of similar individual cells, and is periodic 

 in its occurrence. The apparently invariable presence of a 

 meganucleus and a micronucleus, the active part which the 

 former seems to play in the vegetative and even sensory activ- 

 ity of the adult, the active part which the latter plays in con- 



