CONTRIBUTIONS TO THE ANATOMY OF THE HIRUDINEA, 449 



occurred in the formation of the vascular system. I have studied 

 the thin-walled capillaries in Pontobdella only. The walls are 

 thin, with apparently a sort of cuticularisation upon the outer 

 surface, while upon the inner surface there appears to be a 

 thin layer of protoplasm, with here and there a nucleus em- 

 bedded within it (see fig. 47). 



The connective tissue of Branchellion needs further ex- 

 amination. 



I have now dealt with all the forms of connective and vasi- 

 factive tissue which occur in the adult members of the group 

 which I have examined, and shown that they may all be derived 

 from an indifferent connective-tissue corpuscle — such corpuscle 

 undergoing ecto- or entoplastic, or even ect-entoplastic meta- 

 morphosis. 



We have in the process of the formation of botryoidal tissue 

 an instance of cell-metamorphosis which in all its phases is 

 neither distinctively ectoplastic nor entoplastic. It may in 

 certain cases be solely entoplastic, the vacuolation taking 

 place within the cell, the lumen formed coming into con- 

 nection with the lumen formed in the neighbouring cells, 

 and so giving rise to the vascular channel. On the other 

 hand, cells lying next one another in two rows may undergo 

 metamorphosis at the edges where the rows join, and the vessel, 

 may thus form in an ectoplastic manner; but it is obvious 

 that in the vast majority of cases neither one of these processes 

 alone occurs, but a mixture of the two. This is, in fact, a case 

 where it is not so important to distinguish between the two 

 methods of cell metamorphosis as to confine our attention to 

 the lumen itself. Is this lumen intracellular or is it inter- 

 cellular? We may further consider the general question — is 

 there any well-founded distinction to be drawn between spaces 

 in the animal body with regard to their relations to the cell or 

 cells surrounding them ? It is a subject which requires 

 further investigation — a more exact knowledge of histogenesis 



