CHAPTER V 



THE CHROMOSOMES 



Observation of the minute structure of the nucleus, together with the 

 evidence from experimental work on heredity, has led to the formulation 

 of a hypothesis which can be stated as follows : 



The nucleus — and in particular that part of the nucleus constituting 

 the chromatin or, at least in most phases of the nucleus, indistinguishably 

 bound up with the chromatin — is the seat of the agency which initiates 

 and controls morphogenesis and function, and hence, since the chromo- 

 somes are carried on from one generation to another through the gametes, 

 it is also responsible for the phenomena of heredity. The chromatin 

 appears to act in this respect not as a homogeneous whole, but rather as 

 an aggregate of smaller bodies, each of which plays a different part 

 though the sum of them all is necessary to the general economy of the 

 organism (analogous to the parts played by the lungs, heart, liver and 

 other organs in the higher animals). These smaller bodies, which 

 constitute the lowest order of living units which need be considered for 

 the purpose of this hypothesis, are aggregated during mitosis in linear 

 series into booies of a higher order, the chromosomes. The sum of the 

 chromosomes again forms the nucleus. 



As the haploid number of chromosomes is sufficient to enable a normal 

 individual to develop (cf. facultative parthenogenesis and merogony), 

 each gamete must contain a complete set of all the units necessary for 

 the production of a normal individual of the species. Hence the diploid 

 zygote must contain a double set of these units, i.e. two of each kind. 



While it is impossible completely to separate the discussion of the 

 morphological and physiological aspects of this thesis (for their inter- 

 dependence is the chief evidence of the correctness of both) it is the 

 former aspect that will be the main consideration in this chapter, while 

 the physiological and more theoretical sides will be specially dealt with 

 in the following chapter. 



123 



