116 GENERAL CONSIDERATIONS. 



ferentiate into various kinds as evolution progressed. The 

 differentiation of the protoplasm around the nuclei would 

 proceed hand in hand with that of the nuclei; the result 

 being a gradually increasing complexity in the tissues of the 

 animal. 



The result would be, if the canal system remained complex, 

 — a sponge ; if, on the other hand, the canal system simplified 

 and preserved only one opening, the ancestor of the other 

 Metazoa. 



It is beyond the scope of this paper to discuss the evolution 

 of the mesoderm. I merely throw this out as a suggestion, 

 which is supported by the manner and order of development 

 of the layers in many animals (a peeling off, so to speak, from 

 the ovum : (1) of ectoderm ; (2) of mesoderm ; (3) leaving 

 the endoderm as the remaining central mass), and as a com- 

 pletion of the scheme which I have put forward in discuss- 

 ing the manner of passage from the Protozoa to the 

 Metazoa. 



Finally, I desire again to draw attention to the fact (1) that 

 the formation of mesoderm in Peripatus is essentially a forma- 

 tion of nuclei, which pass to their respective positions and 

 arrange themselves in the protoplasmic reticulum there present; 

 and (3) that the primitive streak is the growing point of the 

 animal, from which almost all the tissues of the body of the 

 adult, viz. ectoderm, endoderm, and mesoderm are formed. 

 This is an important point, to which sufficient attention has 

 not been directed. Almost the whole of the embryo, behind 

 the fifth or sixth somite — not merely the mesoderm, but all 

 the layers — derives its nuclei from the primitive streak. The 

 primitive streak nuclei are therefore not merely mesodermal, 

 but ectodermal and endodermal as well. 



3. The last feature in the development of Peripatus 

 cap en sis which it is necessary to notice in its general bear- 

 ings, is the development of the body cavity and the fate of 

 the coelom. 



The coelom, as is well known, is the term applied to a body 

 cavity with certain characters — characters which may be 



