264 THE BIOLOGY OF BIRDS 



in the inheritance are finding expression, are being actualised ; 

 but we do not understand the process. Development is the 

 making visible of the latent manifoldness of the inheritance. 

 It always implies two things — differentiation and integration. 

 Differentiation is the structural side of division of labour — 

 the appearance of nervous, muscular, glandular, skeletal, and 

 other cells. Integration means regulating and harmonising 

 the whole creature, binding its parts together into a well- 

 controlled unity. 



Development implies a multiplication of cells, an arrange- 

 ment of these in various ways, and then the establishment 

 of the differences between them — some becoming nervous, 

 some muscular, some glandular, some skeletal, and so on. 



The multiplication of cells is usually effected by the 

 familiar but mysterious process of cell-division, in the course 

 of which each of the nuclear bodies or chromosomes (occurring 

 in definite number in each species) is split lengthwise, so 

 that each of the two daughter-cells gets a meticulously 

 exact half of the nuclear equipment of the parent-cell. 



In parts of chick-embryos cultivated in nutritive solutions 

 the process of cell-division has been carefully timed by 

 Warren H. Lewis and Margaret R. Lewis (19 17). A 

 common period was between two or three hours for the 

 complete process, but some cells went through the whole 

 intricate process within one hour. 



The succession of steps in the development is very well 

 known, especially in a familiar type like the chick, but the 

 actual " go " or physiology of the development is very 

 obscure. We sympathise with Harvey's words on " the 

 efficient cause of the chicken " : — 



" Although it be a known thing subscribed by all, that 

 the foetus assumes its original and birth from the male and 

 female, and consequently that the egge is produced by the 

 cock and henne, and the chicken out of the egge, yet neither 

 the school of physicians nor Aristotle's discerning brain 

 have disclosed the manner how the cock and his seed doth 

 mint and coine the chicken out of the egge." 



At a very early stage the developing embryo of the bird 



