PART II. 

 ORGANOGENY. 



INTRODUCTION. 



OUR knowledge of the development of the organs in most of the 

 Invertebrate groups is so meagre that it would not be profitable to 

 attempt to treat systematically the organogeny of the whole animal 

 kingdom. 



For this reason the plan adopted in this section of the work 

 has been to treat somewhat fully the organogeny of the Chordata, 

 which is comparatively well known; and merely to indicate a few 

 salient facts with reference to the organogeny of other groups. In 

 the case of the nervous system, and of some other organs which 

 especially lend themselves to this treatment, such as the organs of 

 special sense and the excretory system, a wider view of the subject 

 has been taken ; and certain general principles underlying the de- 

 velopment of other organs have also been noticed. 



The classification of the organs is a matter of some difficulty. 

 Considering the character of this treatise it seemed desirable to 

 arrange the organs according to the layers from which they are 

 developed. The compound nature of many organs, e.g. the eye and 

 ear, renders it, however, impossible to carry out consistently such a 

 mode of treatment. I have accordingly adopted a rough classification 

 of the organs according to the layers, dropping the principle where 

 convenient, as, for instance, in the case of the stomodseum and procto- 

 dseum. 



The organs which may be regarded as mainly derived from the 

 epiblast are (1) the skin ; (2) the nervous system ; (3) the organs of 

 special sense. 



Those from the mesoblast are (1) the general connective tissue and 

 skeleton ; (2) the vascular system and body cavity ; (3) the muscular 

 system ; (4) the urinogenital system. 



212 



