10 



INVERTEBRATE ZOOLOGY 



shells are frequently developed about the egg cell. The male 

 germ cell or microgamete is usually very minute and represents 

 a specialization for effective locomotion. 



In many kinds of animals, but a single type of germ cell is 

 produced by any given individual. Those which produce macro- 

 gametes or ova are designated as females, while those which pro- 

 duce microgametes or spermatozoa are called males. When 

 both kinds of germ cells occur in the body of the same individual 

 such a one is said to be hermaphroditic. Usually this condition 

 involves the presence of two distinct gonads but in some molluscs 

 there is a hermaphroditic gonad which produces both eggs and 

 spermatozoa. 



Fig. 7. — Two cell stage in development of the egg of Ascaris tncgalocephala, 

 showing chromatin diminution in somatic cells. Germ cells are derived from 

 blastomere marked s which retains all of its chromatin. (After Boveri). 



Early Isolation of Germ Cells. — Long before germ cells become 

 functional they are readily distinguishable from the other cells 

 of the body. The exact point at which this differentiation of 

 germ cells from somatic cells is accomplished has been determined 

 for only a few animals but in these instances the cells destined to 

 form the gametes are distinguishable at a remarkably early period 

 in development of the young. In Ascaris megalocephala, a 

 nematode of the horse, the first division of the fertilized egg (Fig. 

 7) separates two cells only one of which (s) retains all of its 

 chromatin intact. Portions of the chromosomes in the other 

 cell are cast off into the cytoplasm. It is only from the cell with 



