246 GERM-CELL CYCLE IN ANIMALS 



of the germ-cell cycle (Chapter II), with the par- 

 thenogenetic or fertilized egg after the maturation 

 processes have been completed, and to exclude all 

 references to the accessory chromosome until later. 



It may be pointed out first that the number of 

 chromosomes in the cells of any individual of a 

 species is, with few exceptions, constant. Thus the 

 thread worm of the horse, Ascaris megalocephala 

 var. univalens, has two; A. megalocephala var. 

 bivalens, four ; the nematod, Coronilla, eight ; the 

 mole cricket, Gryllotalpa vulgaris, twelve; the 

 bug, Pentatoma, fourteen ; the rat, sixteen ; the 

 sea urchin, Echinus, eighteen ; the salamander, 

 Salamandra maculosa, twenty-four ; the slug, Limax 

 agrestis, thirty-two ; and the brine shrimp, Artemia, 

 one hundred and sixty-eight. This number, however, 

 is reduced one-half during the maturation of the 

 eggs and spermatozoa so that the mature eggs and 

 spermatozoa possess only half as many chromosomes 

 as the other cells in the body ; for example, the body 

 cells, oogonia, and spermatogonia of the rat are 

 provided each with sixteen chromosomes, but the 

 mature eggs and spermatozoa contain only eight. 

 Parthenogenetic eggs differ from those that require 

 fertilization, since in these the complete or diploid 

 number of chromosomes is retained. When cleavage 

 is inaugurated in such eggs, a spindle is formed, the 

 chromosomes are halved, and each daughter cell 

 acquires one-half of each chromosome as in ordinary 

 mitosis. In fertilized eggs, however, the nucleus 

 brought in by the spermatozoon fuses more or less 



