162 GENERAL EMBRYOLOGY 



in connection with the conditions in the higher plants (Strasburger) has 

 led to the suggestion (Whitman) that even in animals the number of 

 chromosomes in the secondary 06- and spermatocytes and mature germ 

 cells, i.e., the haploid number, is again in reality the normal, that this 

 is doubled in fertilization, and remains doubled throughout the somatic 

 divisions, only to be again reduced to normal by the subsequent matura- 

 tion divisions. Upon this hypothesis, which also explains the present 

 precedent relation of maturation to fertilization, the two cell generations 

 immediately preceding fertilization are all that remain of the primary 

 phase of the animal life cycle. 



The alternation between the sporophyte and gametophyte in ferns 

 and mosses is truly an alternation of generations and we may thus see 

 an alternation of generations even in the higher plants where there may 

 be a total of only four divisions with the haploid number the normal 

 according to some. If this is allowed, it is possible that in animals 

 where there are but two of these corresponding cell divisions, we might 

 still speak of an alternation of generations as well ; the equivalent of the 

 gametophyte would then be represented, vestigially, only by the cells 

 with the haploid chromosome group, i.e., primary and secondary 06- 

 and spermatocytes which form the gametes proper and the equivalent 

 of the sporophyte generation would be represented by all the remaining 

 generations of cells which we commonly think of as the true organism, 

 and which forms "asexually" the 06- and spermatogonia the equiva- 

 lents then of the spore mother cells. Of course in animals the matter 

 is complicated greatly by the separation of the two sexes as two sepa- 

 rate individuals. Such a comparison as this must remain, at least for 

 the present, as an interesting speculation merely, for none of the 

 Metazoa offers any variations in the maturation process which shed any 

 light upon the comparison. 



REFERENCES TO LITERATURE 



AGAR, W. E., The Spermatogenesis of Lepidosiren paradoxa. Q. J. M. S. 



67. 1911. 



VAN BENEDEN, E. (See ref. Ch. II.) 

 BOVERI, T., Zellenstudien I. 1887. (See ref. Ch. III.) 

 BRAUER, A., Zur Kenntniss der Spermatogenese von A scaris megalo- 



cephala. Arch. mikr. Anat. 42. 1893. Zur Kenntniss der Rei- 



fung des parthenogenetisch sich entwickelnden Eies von Artemia 



salina. Arch. mikr. Anat. 43. 1894. 

 CALKINS, G. N., and CULL, S. W. (See ref. Ch. II.) 

 CARDIFF, I. D., A Study of Synapsis and Reduction. Bull. Torrey 



Botan. Club. 33. 1906. 

 COE, W. R., The Maturation and Fertilization of the Egg of Cerebratulus. 



Zool. Jahrb. 12. 1899. 



