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 profit- 

 able 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 Chor- 

 data, 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 princi- 

 ples underlying the development 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 

 stomodaeum and proctodaeum. 



The organs which may be regarded as mainly derived from 



