160 GENERAL EMBRYOLOGY 



The second unusual modification of maturation is to be seen in the 

 spermatogenesis of many Arthropods, chiefly Insects. These species 

 have already been mentioned as showing a numerical difference between 

 the chromosome groups of the male and female individuals, the female 

 having, in different species, one or several chromosomes more than the 

 male. In these forms the first maturation division is typical and the 

 two secondary spermatocytes are similar. But the second maturation 

 division is asymmetrical in that one or more chromosomes known as the 

 accessory or idiochromosomes fail to divide and are therefore distributed 

 to only one-half of the spermatids and spermatozoa. Half of the sperm 



cells then have ~ chromosomes, the other half - plus one, or more as 



2i ' 



the case may be in different species. These striking phenomena and 

 their relation to the question of sex determination are described more 

 fully in Chapter VII. 



In conclusion we should mention briefly the place of the maturation 

 divisions in the life histories of different organisms. In any many-celled 

 organism the life cycle as a whole may be said to consist of two phases, 

 one characterized by the possession of the diploid chromosome group, 

 the other by the haploid group. Among all of the Metazoa, and many 

 of the Protozoa, there are invariably only two cell generations with the 

 haploid number, and further, these always are the two final generations 

 in the process of gametogenesis. Here they seem bound up with the 

 process of fertilization and are to be understood only from the point of 

 view of what is involved in this process. Considering these forms alone 

 it is difficult to understand how it should have come about that numerical 

 reduction of the chromosomes should occur in advance of the condition 

 out of which arose the necessity for reduction, namely, the fusion of the 

 germ nuclei. But this arrangement is by no means invariable. In 

 Amceba diploidea (Hartmann and Nagler) reduction does actually occur 

 after conjugation. And in some of the lower plants, such as many of 

 the green Algae (ChlorophyceaB) , the relation between fertilization and 

 numerical reduction is that which apparently must have been the more 

 primitive. In these forms the gametic nuclei contain the same number 

 of chromosomes (s) as do the somatic or vegetative cells; these fuse 

 forming a zygote with double this number (2s). This fusion is then 

 followed immediately by two maturation divisions, the first of which is 

 usually heterotypic, which result in the formation of four cells, each 

 again with the original vegetative number (s). Certain or all of these 

 four cells then produce the body of the new organism, all the cells of 

 which, including the germ cells when these form, have this same chro- 

 mosome number (s) . That is, numerical reduction of the chromosomes 

 follows syngamy, a relation which seems more understandable than the 

 more common precedence of reduction. In describing cases like these 



