REDUCTION IN THE PI A NTS 



195 



reduced number of chromosomes (twelve) appears in the primordial 

 germ-cells which are differentiated in the blastula-stage (Fig. 56). 

 He adds the interesting discovery that in this form the somatic nuclei 

 of the cleavage-stages show the same number, and hence concludes 

 that all the chromosomes of these stages are bivalent. As develop- 

 ment proceeds, the germ-cells retain this character, while the somatic 

 cells acquire the usual number (twenty-four)— a process which, if the 

 conception of bivalent chromosomes be valid, must consist 'in the 

 division of each bivalent rod into its two elements. We have here a 

 wholly new light on the historical origin of reduction ; for the pseudo- 

 reduction of the germ-nuclei seems to be in this case a persistence 

 of the embryonic condition, and we may therefore hope for a future 

 explanation of the process by which it has in other cases been 

 deferred until the penultimate cell-generation, as is certainly the fact 

 m Ascaris} The foregoing facts pave the way to an examination of 

 reduction in the plants, to which we now proceed. 



D. Reduction in the Plants 



Guignard's and Strasburger's observations on reduction in the 

 flowering plants gave a result which in substance agrees with that 

 obtained by Boveri and Brauer in the case of Ascaris. These 

 observers could find absolutely no evidence of a transverse or reduc- 

 ing division, and asserted that the reduction in number is directly 

 effected by a segmentation of the spireme-thread into half the usual 

 number of chromosomes; i.e. by a process exactly corresponding 

 With the "pseudo-reduction" of Riickert (see Fig. 25). These 

 observers find that in the male the chromosomes suddenly appear 

 m the reduced number (twelve in the lily, eight in the onion) at 

 the finst division of the pollen-mother-cell, from which arise four 

 pollen-grains. In the female the same process takes place at the 

 f^rst division of the mother-cell of the embryo-sac. Strasburger 

 and Guignard agree that in the subsequent divisions these chromo- 

 somes do not form tetrads, but undergo simple longitudinal split- 

 ting at each successive division. In case of the male there are 

 at least four of these divisions; viz. two divisions to form the 

 four pollen-grains, a third division to form the vegetative and 

 generative cell of the pollen-grain, and finally a fourth division 

 of the generative nucleus in the pollen-tube. In all these mitoses 

 the reduced number of chromosomes appears, and each division is 

 followed by a return of the nucleus to the resting state. In the 



'It may be recallc-,1 that in .Ascaris Boveri proved that the primordial germ-cells have the 

 full number of chromosomes, and Hertwig clearly showed that this number is retained up 

 to the last division of the spermatogonia. 



