490 ADAM SEDGWICK. 



Stage A, and in the earlier embryos of Stage b^ vanishes later 

 in Stage b, I do not know what becomes of it. 



Of the mesoblastic somites during Stage c there is but 

 little to say. They maintain the same relations and struc- 

 ture (PI. XXXIV, fig. 6 a — d) as in the preceding stage, 

 that is to say their walls are everywhere in contact with the 

 adjacent ectoderm or endoderm. The cells of the somatic 

 layer are thicker than those of the splanchnic layer. This 

 difference was observable in the previous stages (PI. XXXIV, 

 fig. 5 a), and becomes, as we shall see, much more marked 

 in the subsequent stages. During Stage d, somites are still 

 being formed at the hind end from the actively growing tissue 

 of the primitive streak. This stage may be said to mark the 

 close of the formation of new somites, i. e. by Stage e the 

 embryo has acquired its full complement, though the posterior 

 are so small and rudimentary that they are not visible from 

 the exterior. 



Summary op the Development of the Body Cavity, 

 Nephridia, and Generative Organs. 



The elucidation of the further changes in the somites, and 

 the development of the body cavity and heart, has presented 

 some difficulties, — difficulties which have very much delayed 

 the publication of my work, but which I am glad to say I 

 have at length completely overcome. It will, I think, be 

 convenient to explain the terms I shall use, and briefly to 

 summarise the results I have obtained, before proceeding to a 

 detailed description. 



The somites are obviously comparable to the somites of 

 other animals. It is no less clear that the cavity in them is 

 homologous with the cavities of the somites of other types, e. g. 

 other Arthropoda and Annelida. I propose, therefore, 

 to call the system of somite cavities and its derivates the 

 ccelom or enteroccele. 



The system of cavities, on the other hand, which arise partly 

 in consequence of the withdrawal of the ectoderm from the 

 endoderm, and partly secondarily in masses of mesoderm, is 



