CHAPTER I 

 INTRODUCTORY 



THAT living creatures reproduce their kind is a fact which is 

 familiar to us all, but it is the peculiar privilege and province of 

 the embryologist to observe and to reflect upon that marvellous 

 series of changes whereby, out of a germ which is comparatively 

 structureless and unformed, a new organism is developed which 

 is, within the limits of variation, like the parents that gave it 

 birth. 



Development is the production of specific form. From 

 a particular kind of germ only a particular kind of individual 

 will normally arise, though unusual conditions may lead to the 

 formation of an abnormality or monstrosity. Thus, while the 

 germ is the material basis, development is the mechanism of 

 inheritance. The student of heredity seeks to express in terms 

 which shall be as exact as possible, ultimately mathematically 

 exact, the degree of similarity between the offspring on the one 

 hand, and parents and more remote ancestors on the other. 

 The embryologist has under his very eyes the process by which 

 that similarity is brought about, and even when the resemblance 

 shall have been stated with all possible precision, it will still 

 remain for him to give an explanation of those changes whereby 

 the inheritable peculiarities of the species are handed on from 

 one generation to the next. 



Used in the widest sense of the word, development includes 

 not merely the formation of a new individual from a single cell, 

 whether fertilized or not, but also the phenomena of budding 

 and regeneration. In a narrower sense, however, the term is 

 restricted to the first of these processes, and a corresponding 

 distinction is made, however artificially, between Experimental 

 Embryology and Experimental Morphology, when the subject 

 is treated from a physiological point of view. 



JEXKINSOX B 



