24 PROTOPLASM 



idea of a linear arrangement of the genes, or hereditary carrying 

 units. He showed also that the chromosomes are paired — a 

 paternal chromosome with a maternal one. These three funda- 

 mental concepts are the basis of the modern chromosome theory 

 of heredity. 



Certain chromosomes have been singled out as having to do 



with this or that hereditary factor; for example, it was early 



noticed that in some animals there is an accessory chromosome. 



McClung first suggested that the X, or accessory, chromosome is 



associated with sex, a hypothesis now generally accepted. Proof 



J A of this has come from many sides. The 



■ J ■ 1 accessory chromosome is often of different 



J m 1 ^^ shape, as in the fruit fly (Fig. 16), or it may 



4^^^r '•^^^^^ be an extra chromosome resembling the 



m ^% others. This latter condition exists in man 



m ■ where the male has 47 and the female 48 



■^ . r^, r chromosomes. The latest evidence indicates 



Fig. 16.— The four , . , . , , , . 



pairs of chromosomes that scx IS not determmcd by this one 

 and the accessory acccssory cliromosome aloiie, though it does 



chromosome in the 



male Drosophiia. {From Seem to play a primary part. 

 T. H. Morgan.) Qq\\ divisiou, or mitosis, proceeds in a 



similar manner in all cells possessing chromosomes. (These 

 structures are lacking, so it is thought, in the lowest forms of 

 life such as bacteria.) In animal eggs, division is accompanied 

 by the appearance of asters, two starlike bodies which form 

 immediately after the sperm enters the egg (Fig. 17). 



The centrosome is a special body occurring with the asters in 

 animal eggs. It is a minute granule situated at the center of 

 each aster when the cell is in division (Fig. 17). The centrosome 

 has gone through the same vicissitudes in regard to its reality 

 as have most other cell inclusions. Fry declares it to be a coagu- 

 lation center and not a permanent cell structure, while E. B. 

 Wilson sees in it a cell organ of some significance with genetic 

 continuity. As yet we have no definite knowledge of the func- 

 tion of the centrosome. Centrosomes are present in many lower 

 plants but apparently absent in higher forms. 



It is the function rather than the reality of cell parts which 

 troubles us most. R. A. Harper regards all cell inclusions as 

 centers of metabolic activity rather than as cell organs. Thus, 

 the green chloroplast is not so much an organ as a ''place where" 



