CHAPTER IV. 



THE CHANGES FROM STAGE A TO F (CONTINUED). 



The Later History of the Somites and Development 

 OF the Body Cavity. 



The elucidation of the further changes in the somites, and 

 the development of the body cavity and heartj 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 

 coelom or enterocoele. 



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 

 what in other animals would be called a pseudocoele. For this 

 second system of cavities and its derivates I shall therefore use 

 the words pseudocoele or vascular space. 



My results can be summed up as follows : 



1. The adult body cavity comes entirely from pseudocoele. 

 The enterocoele has no part in its formation. 



2. This statement applies also to the heart and pericardium. 

 These are both pseudocoelic, and have nothing to do with 

 enterocoele. 



