MATURATION 



137 



clearly in this form of maturation division (Boveri). In cases 

 oPteTfad formation, when^the chromosomes appear in the 

 primary spermatocyte, after the resting stage, each of the newly 

 organized bivalent elements comes out in the form of four 

 small bodies, the tetrads, arranged approximately in a square 

 (Fig. 73, E). These bodies result from two successive splittings 

 of each chromosome into four columns of granules, each of 



FIG. 73. Tetrad formation in the spermatogenesis of Ascaris megalocephala 

 bivalens. After Brauer. X 795. A-G. Stages in the division of the primary 

 spermatocyte. A, B, splitting, and C, condensation of chromatin thread, seen 

 in side view. D shows, in end view, that the splitting is double. Centrosome 

 divided. E. Migration of centrosomes and formation of spindle. F, G. Separa- 

 tion of the two groups of dyads and division of the cell body. H. Secondary 

 spermatocyte containing two dyads. /. Division of secondary spermatocyte. 

 /. Two of the spermatids, each with two "monads" or single, univalent, chromo- 



which is then condensed into a single element (Fig. 73, A, B, C, 

 D). We may recognize here a precocious division of the chro- 

 mosomes, which in these cases precedes considerably the 

 division of the nucleus and cell as a whole. Not only this, but 

 there are two chromosomal divisions, corresponding with two 



