CHAPTEE I 



INTRODUCTORY : GROWTH. STRUCTURE OF 



THE GERM-CELLS 



EMBRYOLOGY is the study of development, and when used 

 in its widest sense signifies the inquiring into the reproduction 

 of specific form in the individual organism, by whatever means 

 that reproduction may be brought about, whether by the 

 development of a single cell, of a bud, or by the regeneration 

 of a lost part or parts. 



But in a narrower and perhaps commoner sense Embryology 

 is the investigation of the first of these processes, of that 

 series of changes by which there is produced from a single 

 cell a new organism which is like the parents that gave it 

 birth. This cell is usually, though not always, a fertilized 

 egg-cell produced by the union of the two germ-cells, the 

 ovum and the spermatozoon ; while these cells therefore are 

 the material basis, the development of the product of their 

 union is the mechanism, of inheritance. 



To the inquirer into the phenomena of development two 

 methods are open. Either he may observe and describe the 

 sequence of changes in as many forms of animals as possible 

 and in that way Comparative Embryology has been built 

 up or he may, looking upon development as one of the func- 

 tions of the organism, add to observation experiment with the 

 deliberate intention of discovering the causes of each step and 

 so of the whole process, of establishing general laws expressing 

 the relation between the various antecedents given in the 

 initial structure of the germ-cells and in the external environ- 

 ment and their consequents, under which general laws fresh 

 particulars may be subsumed, by which they may be explained, 

 and from which predictions may be made. This is indeed the 



1963 B 



