CHAPTER I 



THE SCIENCE OF EMBRYOLOGY 



I. The place of embryology among the biological sciences 



The core of the science of embryology is the study of developmental 

 phenomena in the early stages of the life-history of animals. It is, however, 

 impossible to discover any general and important dividing line between 

 the embryonic and later stages of development, and there is no good 

 reason to exclude from the purview of the subject those processes of devel- 

 opment which take place in stages later than the strictly embryonic. It is 

 best, in fact, to understand the word 'embryology' as referring to all 

 aspects of animal development, in which case it will include, among the 

 peripheral fields in which it shades off into other sciences, some pheno- 

 mena which may also be considered as parts of endocrinology or of gene- 

 tics. 



From whatever point of view one regards the biological sciences, the 

 study of development will inevitably be found to take a central position 

 among them. If one attempts to view biology as a whole, there are broadly 

 speaking two main approaches which one can adopt; either one tries to 

 formulate a general system which will exhibit all the major aspects of 

 animal existence in their proper relation to one another; or one searches 

 for a theory of ultimate units which could play the same role for biology 

 as the electrons and similar particles do for physics and chemistry. 



From the first, or synthetic, point of view, the most fundamental 

 character of living things is the way in which time is involved in their 

 existence. An animal functions from minute to minute or from hour to 

 hour, in feeding, digesting, respiring, using its muscles, nerves, glands and 

 so on. These processes of physiological functioning may be repeated within 

 periods of time which are short in comparison with the lifetime of an 

 individual animal. But there is an equally important set of processes, of a 

 slower tempo, which require appreciable fractions of the life-history and 

 are repeated only a few times, if at all, during one life-cycle; these consti- 

 tute development. Still longer-term processes are those of heredity, which 

 can only be realised during the passage of at least a few generations and 

 which form the province of genetics. And finally, no full picture of an 

 animal can be given without taking account of the still slower processes 

 of evolution, which unfold themselves only in the course of many life- 



