CONTRIBUTIONS TO THE ANATOMY OF THE HIRUDINEA. 451 



a. Direct, the lumen appearing at once. 



(3. Indirect, the appearance of the lumen being delayed ; the 

 latter to be considered as a " perturbation" of the former process. 



" Direct paracytic coelosis " occurs in the embolic formation of 

 the typical invaginate gastrula or the auditory involution of 

 Vertebrata. " Indirect paracytic coelosis " occurs where a solid 

 mass of cells is pushed in and the space then accumulates 

 between them, as in the formation of the archenteron where 

 there has been an epibolic invagination, and in the formation 

 of all segmentation cavities. 



" Direct endocytic coelosis " takes place wherever vacuola- 

 tion occurs within a single cell as in the developing blood 

 capillary of the Vertebrata, while the process may be termed 

 ^' indirect endocytic coelosis " in a case where it is only after 

 that cell has divided and given rise to a group of cells that 

 endoplastic vacuolation or ectoplastic metamorphosis of each 

 individual cell causes the formation of a lumen common to 

 that group. Such takes place in the formation of the botry- 

 oidal tissue discussed above, and if I surmise correctly in 

 all instances where a solid cord of cells is formed and the 

 central ones subsequently break down forming the lumen 

 (large blood-vessels of Vertebrata, ? all large blood-vessels, ? 

 the lumen of the heart^). 



Thus, in all endocytic coelosis the lumen has necessarily an 

 intracellular origin. Where whole groups of cells, as in the 

 central portion of a solid rod, break down at once, the lumen 

 may appear to be intercellular in origin though really formed 

 by the metamorphosis of cell-substance. On the other hand, 

 in cases where the origin was obviously an intracellular one, 

 the cell in which the lumen was developed may divide and 

 the lumen may be ultimately surrounded by a group of cells, 

 and so may come to be intercellular and indistinguishable in 

 the adult from a paracytic space. Such is the case with the 

 capillaries of Vertebrata: the lumina of the cells run together, 

 the cells divide, and the capillary with its endothelial lining is 



^ See Balfour's account of the formation of the heart in spiders, ' Comp. 

 Anat.,' vol. i, p. 374. 



