THE DEVELOPMENT OF ORGANISMS 725 



a spermatocyte of the penultimate set divides into two spermato- 



cytes with - chromosomes, and each of these divides mitotically 



^ . n 



into two spermatids or young spermatozoa, each with - chromo- 

 somes. Thus each penultimate spermatocyte gives rise to four germ- 

 cells, all viable, each with the reduced number of chromosomes. 

 Here the parallelism with the reduction in ovum maturation is very 

 close, as will be seen; but in the case of the ovum, only one of the 

 four final cells, the ripe ovum itself, is viable. 



R£jC 



MAT 



MAT 



Fig. 113. 



Comparison of Sperm-Development and Ovum-Development. P.S.C., primitive 

 sperm-cell ; with nucleus (N) and its chromosomes (CHR) ; SM, sperm- 

 morula, giving rise to spermatocytes which again divide (SPC' and 

 SPC-). In the penultimate spermatocytic division meiosis occurs, and 

 the number of cnromosomes is reduced to a half. The final spermatocytes 

 develop into spermatozoa (SPZ). PEC, primary egg-cell, with its 

 nucleus and chromosomes; MAT', the extrusion of the first polar body 

 (iPB), effecting meiosis; MAT^ the extrusion of the second polar body 

 (2PB) by mitosis. The first polar body may divide into two. Fi, fer- 

 tilisation of mature ovum, with its reduced female pronucleus (PN), by 

 the entrant spermatozoon with its centrosome (C); F^, fertilised ovum 

 with the normaKnumber of chromosomes restored; CL, cleavage or first 

 division of the fertilised ovum. 



Maturation of Ovum. — Primordial germ-cells or oogonia in the 



ovary multiply by mitotic division and form many oocytes. After 



a period of multiplication, there is a period of growth, and the 



oocytes become larger. They may burst from the ovary at this 



stage, but they have still to go through the process of maturation. 



The nucleus of the unripe ovum moves to the periphery and divides 



meiotically into two, giving off the first polar body, which thus 



n 

 contains - chromosomes. The first polar body, though a sister-cell 



of the ovum, never comes to anything, but it may divide equa- 



