194 



PRINCIPLES OF ANIMAL BIOLOGY 



number, barring differences in the sexes, is constant for the species. 

 In an animal descended from two parents, these chromosomes, with cer- 

 tain exceptions that may for the present be ignored, come in equal num- 

 bers from the father and the mother. Half of the chromosomes in any 

 cell may therefore 'be designated paternal, the other half maternal. 

 These chromosomes may look precisely alike and may in fact be exactly 



^ermatogonia. 



Primary 

 Spermatocyte. 



Dyads. 



Secondary 

 Spermatocyte^ 



Tetrads 

 Reduction 



Spermatozoa 



Primary 

 Oocyte 



Isf Polar Body. 



fSecondary 

 Oocyte 



Dyads. 



^ad Polar Body. 



Mature tqq. 



Pert/I/zed Egg. 



First Ckai/age. 



Fig. 1G4. — Spermatogenesis and oogenesis diagramniatically represented. The black 

 chromosomes may be assumed to be of paternal origin, the white ones maternal. 



alike; the terms paternal and maternal refer only to their source, not to 

 their nature. 



The spermatocytes grow considerably in volume, and at the same 

 time their chromosomes come together in pairs. Each pair is composed 

 of one paternal and one maternal chromosome. The pairing is not a 

 purely fortuitous occurrence, for each paternal chromosome meets a 

 particular maternal chromosome. As a result of this union of the 

 chromosomes there are, of course, half as many pairs as there Avere 

 chromosomes before. 



