1890.] -"-I- [Ryder. 



also be interfered with in other ways. It seems almost self-evident that 

 where karyokineses become simultaneous and rapidly successive there 

 must be a greater inherent probability that variations should be induced 

 through disturbances of the karyokinetic processes. 



Latterly much discussion has taken place regarding rejuvenescence and 

 the relation of the process of fertilization to a supposed renewal of the 

 youth of the sexual cells. It may be suggested that the sexual cells prob- 

 ably never grow old from the causes which act upon the other cells of the 

 body to render them senile, and it may be that the real ground for a theory 

 of rejuvenescence lies not in fertilization itself but in the fact that the 

 sexual cells are functionless and have not been belabored with physiologi- 

 cal duties in the parent body. Where they are produced annually, as in 

 many animals and in all plants, they are also the youngest cells of the 

 parent body, while the spermatozoa, produced in some animals at hourly 

 intervals, are still younger, or more youtbful. The male cell is therefore 

 the most youthful, the least functional and the one most disposed to ex- 

 hibit its activities of growth under favorable conditions with the greatest 

 energy, though not necessarily in the sense that such a display of greater 

 energy would be favorable towards provoking variability, except as pro- 

 vided for by the cytoplasmic field of the ovum or female element. 



It has also been pointed out that the first cleavage of the oosperm cor- 

 responds to the future median plane of the embryo or to the line dividing 

 the future hypoblast from the epiblast. But there are still other relations 

 which connect these phenomena with the fore and aft disposition of the 

 body of the parent. It is a matter of common knowledge that the Infu- 

 soria when undergoing division divide either lengthwise or crosswise. In 

 fixed forms — Vorticella — the division occurs lengthwise of the parent and 

 in conformity to the mode in which the future individual is related to the 

 colony by its base. In many free forms the division is crosswise, and it is 

 a singular fact that the end of the hinder individual next to the posterior 

 end of the anterior one becomes the future anterior end of the hindmost 

 one. These two forms of division have been developed adaptively and in 

 conformity with very different conditions in the two cases. Why should 

 the end of the young Paramoecium next the foremost or parent individual 

 become its anterior extremity preferably to the other one? Does this not 

 indicate that use and habit may have had an influence in giving the plas 

 ma of both a bias which extended to the sflrna of the posterior bud and 

 which expresses itself in this peculiar polar conformity to that of the an- 

 terior parent individual, which is more somatic in its character ? 



Numerous other forms, such as Volvox, illustrate the same tendency of 

 the axis of the young to conform to the axis of the parent. In Fishes the 

 embryos of BatracJms tau, which are attached to a fixed substratum after 

 the rupture of the egg-membrane, by the adhesion of the yolk sack to the 

 latter, show that, at the time of deposit, the future axis of the whole brood 

 of embryos was predetermined in the body of the parent. That this must 

 be so may be concluded from the astonishing fact that the heads and tails 



