PART II.
ORGANOGENY.
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 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 stomodæum and proctodæum.
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.