It is interesting to note that Weismann and Mendel 

 differ in their conception of the constitution of the fertihzed 

 egg. The former believed that all the characters of both 

 parents are represented by ids in the fertilized egg. The 

 latter showed that characters are segregated in the repro- 

 ductive cells, only one of two contrasting characters of the 

 parents being present. 



"A reproductive cell, therefore, contains a selection 

 of characters from its ancestry, and not a complete collec- 

 tion" (Coulter). 



Chapter 16— BIOLOGIC METHOD OF INVESTI- 

 GATING HEREDITY 



In the preceding discussion of Mendelian inheritance, 

 especially in connection with linkage or coupling, "crossing- 

 over" and sex, it was found necessary to assume the exis- 

 tence of chromosomes, as bearers of the heredity factors. 

 This chapter will deal more particularly with the study of 

 chromosomes, which is the third or Biologic method of 

 studying heredity. (See page 46). 



(a)— The Cell in Heredity 



It is known that among the simplest plants and animals 

 every cell has the power to reproduce the entire organism. 

 However, in the higher multi-cellular plants and animals 

 the power of reproducing the organism is restricted to the re- 

 productive or germ-cells. Looked at from this standpoint 

 the germ-cells of the higher organisms are not cells endowed 

 with special powers, but cells that have retained this primi- 

 tive power, belonging to cells of the simplest plants and 

 animals. 



It should also be borne in mind that re^o^ctign^in- 

 volves''ceiraifferenfTationr"arwell as^cell^multigTicat^ . 

 HeredTfy transmissi on is, therefore. cohcernedT^w itircell 

 structurerang"the factors thaTcontrol th emadiineryglhe 

 ceTT P af enlal - li fehess, as well as vanationfis to beex- 

 pMhed'; also the appearance of new characters and the loss 

 of old ones. 



The Cell.— "A cell isabody^consisti^^ 

 proio^ias^'lnl^'W^m^J^mr^^^gJi^I^Si^^^ 

 cWpIas!TT r^nd'^t tYe32icianze^ ^ and lcentrosome 



(In^imaTs), while as unessential accompaniments (espec- 

 ially in plants) ^ay be enumerated : 



129 



