EARLY DEVELOPMENT. 



17 



with great constancy in almost all animals, and, indeed, is probably represented in 

 the development of vegetal organisms as well — has been the subject of much dis- 

 cussion and speculation. The most satisfactory explanation of the significance of 

 maturation has been proposed by Van Beneden, Boveri, and others, based upon the 

 comparison of the changes which take place in the development of the germ-cells 

 of the two sexes. 



In order to appreciate the necessity and the meaning of maturation of the ovum, 

 it will be of advantage to take a brief survey of the phenomena attending the devel- 

 opment of the male sexual elements. The seminiferous tubules of the testicle are 

 lined with epithelial cells, certain of which, known as the primary spermatocytes. 



Fig. 13. 



G ^^1 



Semi-diagrammatic representation of the formation of the polar bodies, based upon observations of invertebrate 

 ova {Ascaris and Physa). n, nucleus; c, c, centrosomes ; s, nuclear spindle; p' , /", first and second polar bodies; 

 ^, egg-nucleus. {After Kostanecki and IVierzejski.) 



increase in size and undergo division, the daughter cells constituting the secondary 

 spermatocytes. Each of the latter, in turn, gives rise to a new generation, the sper- 

 viatids, from which the spermatozoa are directly formed, the chromatin of the sperma- 

 tid being stored within the head, and the centrosome forming the neck-granules within 

 the middle-piece. The spermatozoon, therefore, represents the third generation 

 and corresponds to the mature ovum. 



Turning to the phenomena of maturation, a parallel process is presented, since 

 the ovarian egg, or primary oocyte, divides into two cells, the secondary oocytes, 

 represented by the ovum and the first polar body, each of which receives one-half of 

 the chromatin, notwithstanding that one of the daughter cells, the first polar body, 

 is disproportionately small ; the repetition of division effects a second distribution of 



