THE PHYSICAL BASIS OF HEREDITY. i^g 



in or near the last two divisions of the germ cells pre- 

 vious to their fusion — that is, in the egg — in the di- 

 visions forming the polar bodies, and in the sperm, in 

 the last two divisions of the spermatocyte which pro- 

 duce the four spermatids out of which develop as many 

 mature spermatozoa. The phenomena are exactly homol- 

 ogous in both cases, as has already been pointed out, 

 differing only in the minor details which do not affect 

 the end result. Two peculiar features mark these di- 

 visions off from all the others which precede and follow 

 them. One of these is the absence of an intermediate 

 resting stage between them, the second division follow- 

 ing immediately upon the first without the reconstitu- 

 tion of the chromosomes into the skein stages. The 

 second peculiarity lies in the fact that the chromatin 

 masses (not the individual chromosomes) appear in one 

 half the typical number of the chromosomes in the first 

 division, and are usually arranged in " tetrads," or 

 groups of four rounded, deeply staining bodies connected 

 by linin fibres. These tetrads are always one half the 

 number of the original rod or thread-like chromosomes. 



Thus in Fig. \\, A represents a sperma- 



Reduction in , . , c a ■ •^\^ ^x, 



togonium nucleus of Ascaris with the 

 Ascans. , ° , , . , , . 



four chromosomes, showmg the longi- 

 tudinal splitting preparatory to division. Fig. 1 1, ^, rep- 

 resents an early spindle stage in the division of the 

 primary spermatocyte, in which not four band-like 

 chromosomes, but two tetrads, or chromatin groups of 

 four rounded bodies are found. Fig. ii, C to F, show 

 clearly the further steps in the spermatogenesis. In Fig. 

 II, (7, the tetrads are grouped in the equatorial plate, 

 and in Fig. ii, D, in the closing stages of the first divi- 

 sion into two spermatocytes, each tetrad has divided into 

 two '■'■ dyads ^" which are drawn to the poles, and the 

 division of the cell body follows. Without an interven- 



