144 THE DEVELOPMENT OF THE GAMETES OR SEX CELLS 



chromosomes within the germinal vesicle condense once again, decrease in 

 length (fig. 72F), and assume conditions more typical of the diakinesis stage 

 (figs. 67; 1 19A). The tetrad chromosomes now become visible. Following the 

 latter chromosomal changes, the nuclear membrane breaks down (fig. 1 19A), 

 and the chromatin elements pass onto the spindle of the first maturation divi- 

 sion (fig. 119B). The nuclear sap, membrane, nucleolus, and general frame- 

 work pass into the surrounding cytoplasmic substance (figs. 11 9A; 132A-C). 

 This nuclear contribution to the cytoplasm appears to play an important part 

 in fertilization and development, at least in some species (fig. 132C; the clear 

 protoplasm is derived from the nuclear plasm). 



e. Character of the Meiotic (Maturation) Divisions in the Spermatocyte 

 Compared with Those of the Oocyte 



1) Dependent Nature of the Maturation Divisions in the Female Meiocyte. 



The maturation divisions in the developing male gamete occur spontaneously 

 and in sequence in all known forms. But in most oocytes, either one or both 

 of the maturation divisions are dependent upon sperm entrance. For example, 

 in Ascaris, a nematode worm (fig. 133), and in Nereis, a marine annelid 

 worm (fig. 130), both maturation divisions occur after the sperm has entered 

 and are dependent upon factors associated with sperm entrance. A similar 

 condition is found in the dog (van der Stricht, '23; fig. 115) and in the fox 

 (Asdell, '46). In the urochordate, Styela, the germinal vesicle breaks down, 

 the nuclear sap and nucleolus move into the surrounding protoplasm, and 

 the first maturation spindle is formed as the egg is discharged into the sea 

 water (fig. 116A, B). Further development of the egg, however, awaits the 

 entrance of the sperm (fig. 116C-F). Somewhat similar conditions are found 

 in other Urochordata. In the cephalochordate, Amphioxus, and in the verte- 

 brate group as a whole (with certain exceptions) the first polar body is formed 

 and the spindle for the second maturation division is elaborated before normal 

 sperm entrance (figs. 117C, D; 119D). The second maturation division in 

 the latter instances is dependent upon the activities aroused by sperm contact 

 with the oocyte. In the sea urchin, sperm can penetrate the egg before the 

 maturation divisions occur; but, under these conditions, normal development 

 of the egg does not occur. Normally in this species both maturation divisions 

 are effected before sperm entrance, while the egg is still in the ovary. When 

 the egg is discharged into the sea water, the sperm enters the egg, and this 

 event affords the necessary stimulus for further development (fig. 131). 



2) Inequality of Cytoplasmic Division in the Oocyte. When the first matu- 

 ration division occurs, the two resulting cells are called secondary spermato- 

 cytes in the male and secondary oocytes in the female (figs. 67, 69). The 

 secondary spermatocytes are smaller both in nuclear and cytoplasmic volume. 

 They also form a definite nuclear membrane. Each secondary spermatocyte 

 then divides and forms two equal spermatids. In contrast to this condition 



