EMBRYOLOGY. 



INTRODUCTION. 



EMBRYOLOGY forms a large and important department of 

 Biology. Strictly interpreted according to the meaning of the 

 word, it ought to deal with the growth and structure of organisms 

 during their development within the egg membranes, before they 

 are capable of leading an independent existence. Modern in- 

 vestigations have however shewn that such a limitation of the 

 science would have a purely artificial character, and the term 

 Embryology is now employed to cover the anatomy and physi- 

 ology of the organism during the whole period included between 

 its first coming into being and its attainment of the adult state. 



The subject-matter of the science of Embryology admits of a 

 twofold classification. It may be placed under a series of heads, 

 each dealing either with a special group of organisms, or with a 

 special department of the whole science. If classified in the 

 first of these ways the science will naturally be divided into 

 an Embryology of Plants, and an Embryology of Animals ; each 

 of which admits of further subdivision. In the second way 

 the subject falls under two primary heads ; viz. Physiological 

 Embryology and Anatomical Embryology. 



The present treatise deals only with the Embryology of 

 Animals, and is further confined to those animals known as 

 Metazoa. The science is moreover treated from the morpho- 

 logical or anatomical, rather than from the physiological side. 



B. II. I 



