450 ALi'^IlED GIBBS BOURNE. 



than we at present possess, and the following remarks must 

 be regarded as somewhat speculative. 



All the cells composing the animal body, however complex, 

 proceed from the fertilised ovum, and are modified in various 

 ways from the primitive embryonic cells. It seems to me, 

 however, that we cannot say so much about the spaces which 

 exist in the animal body. 



Some of these, e. g. the contractile vacuoles of Protozoa, the 

 ducts in the nephridial cells of leeches, the newly-developed 

 vertebrate capillary, and all such intracellular spaces, are 

 obviously formed by actual metamorphosis of the cells 

 themselves, and are surely to be contrasted with such spaces 

 as the lumina of invaginated gastrulae, in the formation 

 of which no cell metamorphosis takes place and which are in 

 fact formed outside cells. The former variety of space may be 

 called endocytic the latter paracytic, and we may distin- 

 guish between endocytic coelosis and paracytic ccelosis. 

 A comparatively small number only of existing histological 

 observations are exact enough to enable us to -test the validity 

 of this distinction. To take an instance, it seems to me very 

 probable that there exists an antithesis in this respect between 

 ccelomic and vascular space in many instances, if not as a general 

 rule. In a considerable number of cases true blood-vessels 

 have been stated to develop by a nipping off of ccelomic space, 

 and Biitschli ^ and others consider this to be the rule ; but 

 there is an ever-increasing number of observations of vessels 

 being formed either by vacuolation of single cells or by the 

 breaking down of the central cells of a solid cord, the remnants 

 of the cells floating in the newly-formed lumen becoming in 

 many cases blood-corpuscles. This is endocytic coelosis. 

 The ccelom, on the other hand, is either formed directly by 

 budding from a cavity derived from the archeuterou, or as 

 a space between cells, there being no appearance to suggest 

 that any change in the cells themselves — any cell metamor- 

 phosis — takes place. Both these processes of lumen formation 

 (coeloteis) may be either : — 



1 Butschli, ' Morph. Jubrb.,' 1SS3. 



