406 INTRODUCTION TO CYTOLOGY 



tiated region of the cytoplasm (Keimbahn-plasma) (Fig. 158, B), or 

 by the presence of certain cytoplasmic granules or inclusions (Keimhahn 

 determinants). These latter are ultimately delivered to the definitive 

 germ cells, the nuclei at the same time showing no differences in the germ 

 and soma cells. Although such cases seem to show that "the factors 

 determining what shall become germ cells and what somatic structures 

 apparently exist in the cytoplasm and not in the nuclei' 1 (Child 1915, 

 p. 329), it is nevertheless very significant for the chromosome theory of 

 heredity that only in the germ cells, whatever the cause of their differ- 

 entiation from the other cells of the body, should the chromatin be 

 retained in the complete state in the cases of Ascaris and Miastor. 

 Whatever may be the relation of the chromatin to differentiation, and 

 whatever may be the degree of its independence of the soma-plasm, it 

 is noteworthy that here it is precisely in the germ cells and in the cells of 

 the germ track — the cells especially important in heredity — that the 

 chromatin shows an unbroken continuity from cell to cell and conse- 

 quently from generation to generation. Were the chromosome mechan- 

 ism disturbed in these cells as it is in the somatic cells, or should 

 " diminished" cells regenerate a completely normal organism, a serious 

 obstacle would be in the path of the chromosome interpretation of here- 

 dity as now formulated. The actual behavior of the chromatin in the 

 germ track of Ascaris argues for rather than against the chromosome 

 theory, at least as regards hereditary transmission. 



A much used argument against Weismann's theory of development 

 (ontogenetic differentiation) is found in the phenomenon of regeneration. 

 It is well known that in certain animals and especially in plants a portion 

 of the body consisting solely of differentiated cells may under certain 

 conditions give rise to a complete individual with functional germ cells. 

 Weismann accounted for such regeneration on the basis of an additional 

 hypothesis which stated that during the sorting out of the hereditary 

 units in the process of cell differentiation certain "supplementary 

 determinants" are carried along unaltered, and that later, if occasion 

 arises, these cause the development of the differentiated cells into an 

 organism with all the usual characters. Since in certain cases (Begonia) 

 almost any cell of the body may undergo regeneration into a complete 

 plant, it is evident that all of the body cells must have a "complete" 

 germ-plasm. Hence the distinciton between a germ-plasm limited to 

 cells capable of producing an entire individual, and a soma-plasm present 

 only in somatic cells without such power, becomes of no value. Every 

 cell capable of regeneration — germ cell, meristem cell, or differentiated 

 somatic cell — contains the complete germ-plasm, which appears to be 

 simply the chromatin possessed by all the cells alike. Lack of power to 

 regenerate is not due to a lack of complete germ-plasm but to other 

 conditions associated with the degree of differentiation shown by the 



