414 Mr. William Batp Hardy [April 6, 



Moenkhaus* crossed these two forms, and traced the fusion of the 

 long and short chromosomes in the formation of the hybrid zygote. 

 But when the zygote prepared to divide, the paternal and maternal 

 elements segregated and formed two groups of chromosomes, the one 

 of long and the other of short chromosomes, and in each segment 

 divisioii the paternal long and the maternal short chromosomes 

 reappeared and acted independently. Another case has been fur- 

 nished by Wilson.f In certain groups of insects there is among the 

 chromosomes of the male cells one distinguished by its small size. 

 The total number of chromosomes, instead of being even, is odd : 

 there are thirteen, and this small chromosome is the thirteenth. It 

 is, in point of fact, only half a chromosome, therefore when each of 

 the others divides into two, it does not divide, but passes bodily to 

 one or other of the two new cells. In this way two different kinds 

 of spermatozoa are formed — those which possess the odd half chromo- 

 some, and those in which it is missing— and they are formed in 

 equal numbers. Now, ova fertilised by the former grow into females, 

 those which are fertilised by the latter grow into males. Therefore 

 this particular chromosome is the carrier of the sex character. 



I have stated the theory of the mechanism of heredity as it seems 

 to be developing. A word of caution, however, is necessary. It is 

 quite possible that we are attaching too much importance to the 

 chromosome simply because, owing to the affinity of its substance for 

 dyes we can follow it in the phases of cell history. The rest of^ the 

 nucleus and cell body does not happen to show such constant affinities, 

 and therefore the sense of sight yields no evidence as to their action 

 in ceU division. Yet, so far as we know, the same detailed processes 

 of synthesis and analysis which we can follow in the chromatin 

 substance may divide the units of the rest of the cell in cell division, 

 and guide the half of each unit to its allotted place in the architecture 

 of the new cells. 



The observations of Conklin * upon a curious ascidian ^^^ makes 

 this even probable. The body of this q^^ is built of five kinds of 

 protoplasm recognisably difPerent to sight during life. These are (1) 

 deep yellow, (2) light yellow, (3) light grey, (4) slate grey, (5) clear 

 transparent. Each of these has a separate history ; the deep yellow 

 protoplasm makes the muscular system, the light grey the brain, 

 the clear trans|)arent the skin, and so on. This q^^ therefore is a 

 mosaic, an architecture of different kinds of living matter, which we 

 can detect and follow owing to associated optical differences. Had 

 these been absent, we should liave known as little of the architecture 

 of this egg as we know of that of eggs in general. 



The independent transmission of characters, and the presence in 

 the germ cells of different kinds of living matter, are indisputable 



* Amcr. Journ. Anat. iii. 1904. t Journ. of Exp. Zool. iii. 190G, 

 ♦ Journ. of Exp. Zool. ii.d905. 



