TEXT-BOOK OF EMBRYOLOGY. 



CHAPTER I. 



GENERAL CONCEPTIONS. 



The student of embryology should start with as clear and definite a concep- 

 tion as possible of what he is to gain from his pursuit of that science. If he is a 

 student of biology or of zoology, he must appreciate that knowledge of the laws 

 of development is an indispensable part of what he must master in order to 

 understand those sciences. He must appreciate that it is from the studies of the 

 embryologist that are derived our conceptions of the nature of sex, of heredity, of 

 variation, of differentiation, and many of our most important notions concerning 

 evolution, both of the individual and of the race. He will learn, further, that the 

 embryo illustrates to him with particular clearness the fundamental principles 

 of morphology. If he be a medical student, he will find in embryology first 

 of all the clue to the intelligent comprehension of the anatomy of the adult, a 

 comprehension which he can obtain in no other way, but he will also gain much 

 knowledge of direct practical value as to the embryo and as to the conditions in 

 the adult, acquaintance with which is invaluable in medical practice. And, 

 finally, he will find that it throws a vast light on pathology, both upon the prob- 

 lems of malformations and monstrosities, and also upon the whole question of 

 pathological change in the tissues. 



The best study of embryology, therefore, is that which continually passes 

 beyond the direct observations to the conceptions which they justify and which 

 underlie many important branches of science which are related to, and in a large 

 part dependent upon, embryology. 



The student ought to strive, accordingly, to pass from the direct observation 

 of the specimen to the generalizations, and accustom himself to regard always 

 each special preparation, which may be submitted to his observation, as an illus- 



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