INTRODUCTION TO PART II. 



IN the first part of the text-book, which treated of the fundamental 

 processes of the beginning of development, it was shown how there 

 were formed from the embryonic cells, the descendants of the 

 cleavage-process, several cell-layers : the outer, the middle, and the 

 inner germ-layers, and the intermediate layer which inserts itself into 

 all the interstices between the former. In the further progress of 

 development each of these chief layers, which GAEL ERNST v. BAER 

 has called the fundamental organs of the animal body, undergoes 

 a series of manifold changes, and is in consequence gradually con- 

 verted into the separate organs of the adult body. 



The study of the development of the organs constitutes the theme of 

 the second part of this text-book. 



A division of the extensive material to be presented here is best 

 undertaken with reference to the separate germ-layers from which 

 the various organs are derived, as was first attempted by REMAK 

 in his pioneer work " Untersuchung iiber die Entwicklung der 

 Wirbelthiere." 



But it must be observed at the very outset that the principle of the 

 classification of organs according to the germ-layers can be carried out 

 only with certain limitations. For the completed organs of the adult 

 are ordinarily compound structures, which are not formed out of a 

 single embryonic layer, but out of two or even out of three. Thus, 

 for example, a muscle is developed from the middle germ-layer and 

 the intermediate layer. The teeth arise from the latter and the 

 outer germ-layer ; the alimentary canal with its glands contains 

 elements from three layers, from the inner and the middle germ- 

 layers, as well as from the intermediate layer. When, notwith- 

 standing, these organs are cited as descendants of one germ-layer, 

 it is for the reason that the various tissues are of unequal value 

 in the construction and function of an organ, the important com- 

 ponents being furnished preeminently by one germ-layer. Thus 

 the structure and the function of the liver or the pancreas are 

 primarily determined by the glandular cells which are derived from 



