CHAPTER XII 



THE PHYSIOLOGY OF DEVELOPMENT 



Inheritance, which was discussed in the last chapter, is a 

 rhythmical repetition in each species of a definite and (on the 

 whole) similarly repeated series of events in which the pro- 

 duction of one individual leads up to the formation of gametes 

 whose union initiates a new being. The fertiHsed egg bears 

 within it the power to develop into an individual resembling 

 the parents from which the sperm and egg were derived. 

 Fertilisation starts in the egg a period of active cell-division. 

 In the initial stages of cleavage all the cells may be, and often 

 are, for a considerable period very much alike. As they go 

 on dividing they differentiate individually and regionally to 

 build up the structural architecture of the new individual. 

 In the early stages there is no increase in size ; at some point, 

 however, the developing organism begins to augment in weight 

 and volume. This process usually goes on long after the final 

 morphological order characteristic of the individual is 

 completely established. Developmental phenomena may thus 

 be considered under two headings : diiferentiation, individua- 

 tion and growth. 



Individuation, or the differentiation of structural pattern 

 in cellular animals, raises perhaps the most recondite issues 

 in the whole field of biological inquiry. It is convenient to 

 consider it separately in its spatial and chronological aspects, 

 that is to say (i) the agencies which determine whether a 

 particular region is to differentiate into one type of structure 

 rather than another ; and (2) the agencies which determine 

 the orderly sequence in which the differentiation of one 

 structure follows another. In this chapter no attempt will 



