INHERITANCE 



297 



sperm\ ^ egg 



\ / 



z 



\ \ I I 



pb 



Pig. 177. Diagram of the derivation 

 of the sex cells (after Boveri). z, 

 the fertilized egg (zygote) ; som, 

 the body plasm (soma); t, the de- 

 velopmental period during which 

 the germ plasm and the body plasm 

 are indistinguishable; sp, sperm- 

 ary; ov, ovary; p, primordial germ 

 ceils; M, the period of rapid in- 

 crease in number and diminution 

 in size (the number of divisions is 

 much greater than shown) ; v, the 

 period of increase in size with dif- 

 ferentiation of cytoplasm; w, the 

 two maturation divisions ; pb, polar 

 bodies; e, egg. 



ing this period. Then follows 

 a period of maturation, or 

 ripening of the sex cells, which 

 involves two successive divi- 

 sions only, and during which 

 the germ cells are known as 

 spermatocytes or oocytes. 

 The four cells resulting from 

 these two divisions become 

 the sex cells, eggs or sperms, 

 but there is one marked differ- 

 ence, indicated in the accom- 

 panying diagram (fig. 177). 

 In the case of spermatocytes, 

 the divisions are equal, and 

 four sperms result ; but in the 

 case of the oocytes, the divi- 

 sions while equal with respect 

 to nuclear parts, are very un- 

 equal with respect to cyto- 

 plasm, one cell retaining nearly 

 all of it, the others being cast 

 out from it as the so-called 

 polar bodies; therefore, but 

 one functional and perfect Qgg 

 results. 



Such are the form changes 

 undergone by the germ cells 

 during their development, 

 among the higher animals in 

 which they have been most 

 carefully studied. They are 

 not to be considered as occur- 

 ring at one time only and in a 



