The Cellular Basis 



179 



FrttilitaKon 

 Union of 

 Simplex f!rovpi 



OO ABCD 



Clrnenge 

 Duplex Oroupt 

 ASCD abed 



aBCd 



Aa.Bb Ce. Dd. 

 By*apti$ 



AbeD 

 Reduction Germ Cell* 



Division Simples Group, 



Somatic Divitioni 

 Duplex Group* 



Fig. 60. Diagram of Germ Cells Corresponding to Fig. 59, show- 

 ing the union of maternal chromosomes (ABCD) and paternal ones 

 (abed) in fertilization, their distribution in cleavage, their union into 4 

 pairs (Aa, Bb, Cc, Dd) in synapsis and the separation of the pairs in the 

 reduction divison. Only 2 of the 16 possble types of germ cells are shown 

 at the lower right. (After Wilson.) 



normal the larva shows abnormal characters (Boveri) ; sex de- 

 termination is associated with the distribution of a particular 

 chromosome to one-half of the spermatozoa, and the fertilization 

 of the egg by one or the other type of spermatozoa. (Wilson). 

 There are many parts of a germ cell, all of which may be con- 

 cerned in heredity and development, but the chromosomes appear 

 to be the seat of the differential factors for Mendelian characters. 

 On the other hand it has been objected by certain investigators, 

 notably by Child, Foot and Strobell, that chromosomes are not the 

 causes of anything, but that they are the "results of dynamic 

 processes," "the expression rather than the cause of cell activi- 

 ties." This objection seems to confuse the idea of natural cause 

 with that of final cause. Science knows nothing of the latter; 

 any natural cause is only a link in the chain of cause and effect, it 



