CHAPTER VI 



HEREDITY AND MORPHOGENESIS 



In this chapter we have to consider the thesis that the nucleus and 

 not the cytoplasm is the substratum by which hereditary qualities are 

 transmitted from parent to offspring, and the correlative hypothesis 

 that it is the nucleus which is the initiator and controller of the activities 

 of the cell and especially of morphogenesis. It is obvious that these two 

 theses are mutually interdependent, and if either were established the 

 other would follow as a corollary without the necessity of further proof. 

 In the meantime, any evidence obtainable for or against the one is in 

 equal degree evidence for or against the other, and therefore the two 

 may be considered together. 



Both these theses are capable of expansion far beyond the limits 

 of a volume of this scope, and have, moreover, so often been made the 

 subject of special treatises that they can only be dealt with briefly here. 



The student must also reaUze that in this chapter the amount of 

 theory bulks larger in proportion to the amount of fact than in the 

 preceding chapters, and therefore that it is all the more necessary for 

 him to keep in mind the proper scientific spirit which is always ready 

 to modify its ideas when the discovery of new facts makes this necessary. 



The principal threads of evidence under this head may be classified 

 as follows : 



(a) The equality of inheritance from male and female parents. 



{b) The process of mitosis and its implications. 



(c) The process of meiosis. 



(d) The parallel which exists between chromosome beha\iour and 



the results of breeding experiments. 



(e) The case of sterile and partially sterile hybrids. 

 (/) Morphogenesis and mode of action of the nucleus, 

 (g) Chromidia and chondriosomes. 



We wil consider these points in order. 



153 



