EMBKYOLOGY, 



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 develop- 

 ment within the egg membranes, before they are capable of leading an 

 independent existence. Modern investigations 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 physiology 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 two- 

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

 dealing either with a special group of organisms, or with a special de- 

 partment 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 sub- 

 division. 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 morphological or anatomical, 

 rather than from the physiological side. 



The marvellous phenomenon of the evolution of a highly 



complicated living being from a simple undifferentiated germ in 



which it needs the aid of the most modern microscopical appliances 



to detect any visible signs of life, has not unnaturally attracted the 



R. E. 1 



