CHAPTER X 



Origin of the germ cells in nralticellular animals Maturation of the germ 

 cells Eeduction of the chromosomes Sex determination in insects 

 Different forms of gametes Mutual attraction of the gametes 

 Fertilization and parthenogenesis. 



IN many multicellular animals the distinction between somatic 

 cells and germ cells becomes manifest at a very early stage in the 

 development of the individual. An extreme instance of this is 

 seen in the parasitic round-worm of the horse, Ascaris megalo- 

 cepliala. Here the distinction in question precedes all other 

 histological differentiation. The two cells or blastomeres into 

 which the fertilized ovum first divides (Fig. 35, C, D) are originally 

 similar to one another, but as they prepare for the next mitotie 

 division of the nucleus a remarkable difference is, according to 

 the observations of Professor Boveri, established between them. 

 Both at first (in the case of the variety known as univalens) 

 exhibit two elongated chromosomes (the variety bivalens, which ia 

 representeST in Fig. 35,having four), but in one the thickened 

 ends of the two chromosomes are thrown off into the surrounding 1 

 cytoplasnTj^vhere IheyTfegenerate, while the mofe~slender middle 

 portions break up. into a number of short pieces. Thus two 

 differentiated cells are produced, one with two large chromosomes 

 and the other with numerous small ones. The latter gives rises 

 by its subsequent divisions to somatic cells only. The former is 

 a primordial germ cell ; for some five or six times it will divide 

 like its parent cell into a somatic cell and a primordial germ cell, 

 but after these early divisions the primordial germ cells will 

 give rise to their own kind only, until the time comes for the 

 production of the actual gametes. The somatic cells, on the 

 other hand, will gradually become differentiated into all the 

 various tissue cells of the adult. 



Perhaps the most significant part of this remarkable process 

 as observed in Ascaris is the elimination of chromatin material 

 from the nuclei of the somatic cells when these are firgi 



B. K 



