Strasburger • The Periodic Reduction of the Number of the Chromosomes 219 



fact that the number of chromosomes 

 characteristic of the generative nuclei 

 of angiosperms, is determined in the 

 mother-cells of the embr}'0-sacs. The 

 investigations of the zoologists have 

 also shown that this determination is 

 effected in the mother-cells of the ova 

 and of the spermatozoa of animals, by 

 two successive cell-divisions, which 

 give rise, in the one case, to four sper- 

 matozoa, and in the other, to the 

 ovum and to three so-called polar 

 bodies. 



But what is the significance of this 

 reduction in number of the chromo- 

 somes in the sexual cells, and of the 

 equality of their number in the male 

 and female cells? The physiological 

 utility of the arrangement readily sug- 

 gests itself: for were it not so, the 

 number of chromosomes in the nuclei 

 of each generation would be twice as 

 great as in the preceding; and again, 

 by this means each parent is repre- 

 sented in the offspring by an equal 

 number of chromosomes, and thus 

 equally transmits its hereditary char- 

 acters. The morphological cause of the 

 reduction in number of the chromo- 

 somes and of their equality in number 

 in the sexual cells is, in my opinion 

 phylogenetic. I look upon these facts as 

 indicating a return to the original gen- 

 eration from which, after it had at- 

 tained sexual differentiation, offspring 

 was developed having a double num- 

 ber of chromosomes. Thus the reduc- 

 tion by one-half of the number of 

 the chromosomes in the sexual cells 

 is not the outcome of a gradually 

 evolved process of reduction, but rather 

 it is the reappearance of the primitive 

 number of chromosomes as it existed 

 in the nuclei of the generation in which 

 sexual differentiation first took place. 

 Viewed from this standpoint, many 

 facts become more readily intelligible: 

 for instance, the immediate and sudden 

 occurrence of the reduction, the de- 

 velopmental stage at which it takes 



place, and the varying length of the 

 interval which separates it from the 

 sexual act. 



The number of chromosomes de- 

 termined in the mother-cells of the 

 pollen of the angiosperms persists up 

 to the formation of the spermatic nu- 

 cleus in the development of this nu- 

 cleus: two divisions take place in the 

 mother-cell resulting in the formation 

 of four pollen-grains; then there is the 

 division in the pollen-grain bv which 

 the generative and the vegetative cells 

 are respectively formed; and, finallv, 

 there is the fourth division, the divi- 

 sion into two of the generative cell in 

 the pollen-tube. The number of chro- 

 mosomes determined in the mother- 

 cell of the embr}0-sac persists through 

 a series of divisions, the number of 

 which varies with the species of plant, 

 until it attains functional importanrc 

 in the ovum. As a rule, the mother-',.-cll 

 of the embr}'o-sac divides twice, and 

 the lowest of the resulting daughter- 

 cells develops into the embr}'0-sac. In 

 the embryo-sac three divisions succeed 

 each other before the nucleus of the 

 ovum is formed. In this case five 

 divisions, and not four as in the de- 

 velopment of the spermatic nucleus, 

 intervene between the reduction and 

 determination of the number of the 

 chromosomes, on the one hand, and 

 the constitution of the sexually func- 

 tional nucleus on the other. 



The reduction of the number of 

 the chromosomes in the pollcn-mother- 

 cells of angiosperms, which has been 

 adduced as an example, is therefore 

 not to be regarded as a preparation for 

 the sexual act; it really marks the be- 

 ginning of the new generation which 

 comes into existence with the primi- 

 tive number of chromosomes. Tliis 

 primitive generation has, however, un- 

 dergone great limitation before it at- 

 tained the reduced ontogeny which it 

 now exhibits in the angiosperms. In 

 the first place it developed sexual di- 



