V ASCARIS 195 



(2) As has already been indicated it is a general characteristic of 

 heredity that, upon the average, the inherited qualities of the individual 

 are derived equally from the two parents. If therefore inherited qualities 

 have a material basis this must consist of a substance derived in approxi- 

 mately equal amounts from the two parents. Ascaris has shown us that 

 there is one such substance (and there is no evidence of any other) namely 

 the substance of the ordinary chromosomes. We are therefore justified 

 in the belief that this substance is the material basis or " vehicle " of 

 heredity. It is necessary that we should guard ourselves from forming 

 too definite a conception of this chromatin as a substance of fixed chemical 

 and physical constitution. We should regard chromosomes primarily 

 as portions of the living substance or protoplasm in which certain living 

 activities are concentrated. The physical and chemical features — staining 

 properties, high refractive index and so on — which make the chromosomes 

 recognizable by the sense of sight are to be regarded as relatively super- 

 ficial accompaniments. 



(3) Syngamy, involving the union of two nuclei, is necessarily accom- 

 panied by meiosis, to keep the chromosome number constant and prevent 

 it from being doubled at each successive syngamy. 



(4) Syndesis, the temporary pairing of homologous chromosomes, is 

 apparently an essential part of the process of meiosis. Its primary 

 meaning is probably to make use of the existing mechanism of mitosis 

 for transferring entire chromosomes — instead of split halves — to the 

 two daughter cells, thus ensuring that the latter shall contain only the 

 haploid number. While we have there the probable primary meaning 

 of syndesis it is well to bear in mind that in such a case as Ascaris where 

 the two homologous chromosomes lie in close apposition side. by side 

 their proximity to one another may. provide the opportunity for possible 

 exchange of substance between the two chromosomes, or for the exercise 

 of mutual influence in more obscure ways. 



(5) The fact that maternal and paternal chromosomes are alike dis- 

 tributed during the course of development to every cell in the body is 

 an important bit of confirmatory evidence to the conclusion, already 

 stated under (2), that the substance of these chromosomes affords the 

 material basis for heredity. 



(6) In the process of mitosis^ which is the almost universal mode of 

 nuclear division all through the animal kingdom, perhaps the most 

 characteristic feature is the remarkable longitudinal splitting of the 

 chromosomes. In fact we might define the mechanism of mitosis as a 

 mechanism for the accurate splitting of the chromosome into two halves 

 and the distribution of these two halves to the two daughter cells. 



