404 INTRODUCTION TO CYTOLOGY 



their offspring (by means of the gemmules). According to the modern 

 view [Mendel; Galton; Weismann], however, children resemble their 

 parents not, strictly speaking, because the latter have passed something 

 on to them, but because both have been produced from the same 

 germ-plasm" (p. 91). "The parent is rather the trustee of the germ- 

 plasm than the producer of the child" (Thomson 1913). 



Weismann attempted further to account for the variations effective 

 in evolution on the basis of his theory of Germinal Selection. He sup- 

 posed that the determinants, while multiplying in the germ cells, are 

 subject to selection like all other organic units. Some determinants, 

 being better placed with respect to the nutritive conditions, are favored 

 thereby and grow stronger and more influential, while others undergo 

 changes in the opposite direction. The cells or parts of the organism 

 receiving the determinants which have had the advantage in the struggle 

 become better developed than those receiving the weaker determinants. 

 As this process continues from generation to generation the new variation 

 gradually increases until it becomes pronounced enough to be laid hold of 

 by natural selection. In this manner Weismann accounted for the 

 preservation of small variations not yet of selective value, and for 

 continued variation along definite lines (orthogenesis) in both plus and 

 minus directions. Thus for him selection was the cardinal principle 

 which ruled not only over organisms, but also over cells, ids, deter- 

 minants, and biophores. As he himself stated it, "This extension of the 

 principle of selection to all grades of vital units is the characteristic 

 feature of my theories." 



Some Modern Aspects of Weismannism. Although the distinction 

 between soma cells and germ cells is not now drawn so sharply as in the 

 days of Weismann, it is nevertheless of interest to note certain facts 

 adduced in support of his contention that the germ-plasm is continuous. 



In Ascaris megalocephala (Boveri 1887c, 1889, 1891, 1892, 1904; 

 Zacharias 1913) it is observed that at the second cleavage mitosis the 

 chromsomes in one blastomere remain entire, while in the other blastomere 

 they become broken up into smaller pieces, some of which are lost in the 

 cytoplasm and are not included in the daughter nuclei (Fig. 158, A). 

 This process is called "chromatin diminution." At the third and fourth 

 cleavage mitoses a similar diminution occurs in all the blastomeres 

 but one; in this one the chromosomes remain entire. At the fifth division 

 it is seen that in the one undiminished cell no further diminution occurs 

 as it divides, and its descendants become the germ cells. The primary 

 germ cell is therefore set apart at the fourth mitosis; and, whereas the 

 other embryonic cells giving rise to somatic structures have undergone a 

 diminution, the entire chromatin outfit is delivered to the germ cells 

 through the undiminished cells of the germ track. A similar condition 

 is present in Miastor (Kahle 1908; Hegner 1912, 1914). In Ascaris 



