XII.] CONJUGATION AND SEXUAL REPRODUCTION. 121 



the latter body generally splits into two secondary polar bodies, 

 and the significance of this apparently aimless division has 

 hitherto been sought in vain. But now we see that it depends 

 on the persistence of a phyletic stage of development, on the 

 survival of an earlier condition, in which the original egg-cells 

 underwent a ' reducing division,' like that of the spermatozoa, 

 producing four cells, each of which was potentially an ovum. 



Moreover in another, and obviously a decisive point, the ' re- 

 ducing divisions' of ova and spermatozoa are in correspondence ; 

 —in the manner and method of the division of the nuclear rods 

 in the daughter-nuclei. The process of karyokinesis here differs 

 from any other mode of nuclear division, in that there is no 

 longitudinal splitting or doubling of the nuclear rods, bringing 

 about a contribution from each rod in the equatorial plate to both 

 daughter-nuclei ; instead of this, half the whole number of rods 

 passes to one pole of the nuclear spindle, and half to the other. 

 Furthermore, there is no resting-stage between the two divi- 

 sions, during which the rods break up into the nuclear network, 

 but the two divisions follow each other without any interval. If 

 the ' reducing division,' for which I have argued, has any exist- 

 ence, we must look for it here ; for, so far as proofs can be 

 afforded by observation, they are forthcoming. The number 

 of nuclear rods is reduced to half, and hence the mass of the 

 nuclear substance is certainly halved. And if we must concede 

 that the rods in a nucleus are not absolutely alike, but are 

 derived from the differing germ-plasms of various ancestors 

 (viz. that the rods consist of such different kinds of germ- 

 plasm), it follows that a reduction of the ancestral germ-plasms 

 is admitted. 



The new facts discovered by O. Hertwig leave only one point 

 obscure. We see indeed that, in the case of the spermatozoon 

 as in that of the ovum, the nuclear rods are reduced to half, but 

 we ask in vain why two successive divisions are necessary to 

 bring about this reduction, when it seems that a single one 

 would suffice. I had formerly concluded that since partheno- 

 genetic eggs expel only one polar body, instead of the two which 

 separate from all ova requiring fertilization, the first division 

 must have a different significance from the second. I regarded 

 the second division alone as the ' reducing division,' and this was 

 a perfectly sound and logical conclusion, so long as it remained 



