4 PRINCIPLES OF EMBRYOLOGY 



times. From this point of view, then, embryology takes its place between 

 physiology on the one side and genetics on the other. 



As a matter of historical fact, the biological sciences at the two ends of 

 the time-scale — those of physiology in the broad sense on the one hand, 

 and of evolution on the other — have been more thoroughly developed 

 than the two sciences of embryology and genetics which come between 

 them. The volume of information available about physiological pheno- 

 mena is immense; their relevance to medicine and animal husbandry has 

 given them practical importance, and the relative ease with which they 

 can be envisaged in physico-chemical terms has made them seem intellec- 

 tually attractive. The study of evolution, which was until recently only 

 shghtly less voluminous, derived its impetus from the feeling that Darwin's 

 work has provided the essential thread which was needed to link all 

 aspects of biology together. Between these two huge masses of biological 

 science, embryology and genetics are rather in the position of the neglec- 

 ted younger sisters in a fairy tale. 



At the present time it looks rather as though the fairy tale will have the 

 conventional ending, and the elder sisters fmd themselves in difficulties 

 from which the younger ones will have to rescue them. This is becoming 

 most apparent in connection with evolutionary studies; their enormous 

 expansion in the past has been mainly by the essentially non-experimental 

 methods of comparative anatomy and taxonomy, and it is already clear 

 that httle progress can be made towards an understanding of the causal 

 mechanisms of evolution v^dthout the aid of genetics and to a lesser extent 

 of embryology. And even physiology finds itself more and more led to 

 the recognition that structural considerations are of the utmost importance 

 for the functioning of biological systems; and this realisation brings it 

 into close contact with embryology, which of all the biological sciences 

 is most concerned with questions of structure and form. 



The central position of embryology is perhaps better appreciated when 

 one regards biology from the other viewpoint, which seeks to discover 

 some category of ultimate units. It is clear that the unit which underlies 

 the phenomena of evolution, and of the short-term heredity which consti- 

 tutes genetics in the narrow sense, is the Mendelian factor or gene. But 

 any theory based on our present knowledge of genes has perforce a most 

 uncomfortable gap in it at the place where it should explain how they 

 control the characters of the animals in which they are carried. For physio- 

 logy, the basic unit is the enzyme. We know that the formation of most, 

 if not all, enzymes is controlled by genes ; in fact it is not unplausible to 

 suggest that genes are simply a particularly powerful class of enzymes. 

 But here once again we fmd ourselves confronted with that most lament- 



