HISTORICAL CONSIDERATIONS 215 



nitely established by later workers and it has become known as Van 

 Beneden's Law. 



(5) Boveri, 1887 and the following years, further established the fact of 

 Van Beneden's Law and also demonstrated that half of the chromo- 

 somes of the cells derived from the zygote are maternal and half are 

 paternal in origin. (Fig. 133 is derived from Boveri's study of Ascaris.) 

 In '00 and '05 he emphasized the importance of the centrosome and 

 centrioles and presented the theory that the centrosome contributed 

 by the sperm to the egg at the time of fertilization constituted the 

 dynamic center of division which the egg lacked; hence, it was a causal 

 factor in development. This latter concept added new thinking to the 

 fertilization problem, for O. Hertwig, 1875, had suggested that the 

 activation of the egg was due to the fusion of the egg and sperm 

 nuclei. The centrosome theory of Boveri eventually became one of 

 the foremost theories of egg activation (see end of chapter). 



(6) During the last five years of the nineteenth century, intensive studies 

 on artificial activation of the egg (artificial parthenogenesis) were 

 initiated. This matter is discussed on page 217 in the section dealing 

 with artificial activation. 



(7) Another attack on the problem of fertilization and its meaning had 

 its origin in the "idioplasm theory" of Nageli. This theory (1884) 

 postulated an "idioplasm" carried by the germ cells which formed 

 the essential physical basis of heredity. A little later O. Hertwig, 

 Kolliker, and especially Weismann, identified the idioplasm of Nageli 

 with the chromatin of the nucleus. In the meantime, Roux emphasized 

 the importance of the chromatin threads of the nucleus and stated 

 that the division of these threads by longitudinal splitting (separation) 

 during mitosis implied that different longitudinal areas of these threads 

 embodied different qualities. (See Wilson, '25, p. 500.) In harmony 

 with the foregoing ideology and as a result of his own intensive work 

 on maturation and fertilization in Ascaris and also upon other forms, 

 Boveri came to the conclusion in 1902 and 1907 (Wilson, '25, p. 916) 

 that development was dependent upon the chromosomes and further 

 that the individual chromosomes possessed different qualities. 



(8) As a result, the field of biological ideas was at this time well plowed 

 and ready for another important suggestion. This came in '01 and '02 

 when McClung ofi'ered the view that the accessory chromosome de- 

 scribed by Henking (1891) as the x-chromatin-element or nucleolus 

 was, in the germ cell of the male grasshopper, a sex chromosome 

 which carried factors involved in the determination of sex. McClung 

 first made this suggestion and stated a definite hypothesis, immediately 

 stimulating work by others; in a few years McClung's original sug- 

 gestion was well rounded out and the dimplete cycle of sex chromo- 



