46 GERM-CELL CYCLE IN ANIMALS 



primary spermatocyte is seen to be divided into 

 four parts,, thus forming in this case two tetrads (C). 

 During the division of the primary spermatocyte, 

 as shown in /), E, and F, half of each tetrad, or two 

 dyads, passes to each daughter cell. The division 

 of the daughter cells, which are known as secondary 

 spermatocytes {G H), results in the separation of the 

 two parts of each dyad so that each of the four 

 spermatids {H) receives one member of each original 

 tetrad or two monads. Thus the chromosomes 

 (monads) of the spermatids {H) are already formed 

 in the primary spermatocytes (C) by two divisions ; 

 whereas the nuclear and cell divisions do not occur 

 until later. The spermatids {H), which proceed 

 to metamorphose into spermatozoa, possess, there- 

 fore, only two chromosomes, i.e., one-half of the 

 number present in the spermatogonia {A) and so- 

 matic cells. 



Tetrad formation does not occur in most animals ; 

 but usually the members of the bivalent chromosomes 

 become separated on the first maturation spindle, 

 the pairs appearing U-, V-, or ring-shaped, as in 

 Fig. 6^. Each secondary spermatocyte receives 

 one-half of each haploid, bivalent chromosome. The 

 second maturation mitosis then ensues, during which 

 each daughter cell is provided with one-half of each 

 chromosome as in ordinary mitotic division. Be- 

 cause of the peculiar behavior of the chromosomes 

 the first division is often called the heterotype, 

 whereas the second is known as the homotype divi- 

 sion. The final results are the same whether tetrads 



