GAMETOGENESIS 475 



and are regarded by some as (lie rnw material for mitochondria, to which 

 latter certain investigators have attached hereditary significance. 



The further description of spermatogenesis will he confined to that 

 half of the secondary spermatocytes containing the accessory or sex 

 chromosome; conditions in the other half are closely similar except for 

 the absence of the odd chromosome. Figs. 21 and 22 illustrate suc- 

 cessive prophase stages. Figs. 23 and 24 illustrate side and polar 

 views of the metaphase of the second maturation mitosis; in the latter 

 all 12 chromosomes are shown. Plate C, Figs. 25 and 26 show anaphase 

 and telophase stages respectively. Fig. 27 is the early spermatid stage, 

 the accessory still conspicuous. A conspicuous cytoplasmic element of 

 these stages is the idiozome (i), apparently derived from a portion of 

 the disappearing spindle. Later stages of the metamorphosis of the 

 spermatid are shown in Figs. 28 to 31. The idiozome divides into two 

 moieties (Fig. 29). These elongate (Fig. 30) and spread out over a 

 filament (Fig. 31), which has grown out from a dark-staining granule 

 on the outer surface of the nucleus. The cytoplasm also concurrently 

 flows backward around this filament. In conformity with what is 

 known of a similar granule with a similar function in various forms, 

 this granule represents the centrosome of Fig. 23, but its origin and 

 history are obscure. Further steps in the metamorphosis are illustrated 

 in Figs. 32 to 3G. The mature spermatozoon consists of a long, rela- 

 tively lightly chromatic, lance-shaped head, a deeply-staining thimble- 

 shaped middle piece (body], and a long gradually tapering tail. The 

 latter consists of the central filament ensheathed by the products of 

 the idiozome. The entire spermatozoon is furthermore enveloped in a 

 close-fitting sheath of cytoplasm. The head originated from the nucleus, 

 and the middle piece presumably from the centrosome and a small 

 contribution of cytoplasm. 



Determination of Sex. Anticipating somewhat, we may say that the 

 process of oogenesis is essentially similar to that described for spermato- 

 genesis. The cardinal difference relates to a numerical inequality between 

 the functional descendants of a spermatogonium and an oogonium : in the 

 former case the four resulting cells are all functional, and from the stand- 

 point of their numerical chromosome content of two sorts; in the latter, 

 three of the resulting cells are abortive (polocytes), only one remaining 

 functional. Of the two classes of spermatozoa, one has an even number of 

 chromosomes, the other an odd number. All mature eggs have the same 

 number of chromosomes as that of the spermatozoa with the greater num- 

 ber. This description applies strictly only to those instances where an odd 



