418 PROFESSOR LANKESTER. 



)naintained, and does, I believe, to this day, that the coelom 

 originated ancestrally by splitting between the deron with 

 its hypoderic cell-layers, and the enteron with its hypenteric 

 cell-layers, and that it had nothing to do with " gastro- 

 vascular outgrowths/' In my paper on the " Invaginate 

 Planula of Paludina,'' just referred to, I gave the first expla- 

 nation of the coelom as uniformly derived from the enteron, 

 and traceable to " gastro-vascular'^ cavities. This view is 

 entertained by Professor Huxley ('The Anatomy of Inverte- 

 brated Animals,' p. 686), though he makes reservations in 

 favour of a schizocoelous condition for llotifera and Polyzoa, 

 It appears to me to be unnecessary to admit a schizocoelous 

 origin of the coslom in any case. It is uniformly developed 

 from parenteric growths, and under extreme modifica- 

 tions the essential features of its ancestral relations can 

 be traced in the most exceptional cases. Thus there are 

 not a few developmental histories in which, as a purely 

 embryonic phenomenon, liquid accumulates between the 

 invaginated enteron and the vesicular deron or ectoderm. 

 This cavity is really only a developmental feature, part of the 

 non -historic mechanism of growth, like the blastopore, and 

 is a continuation of the pseudo-blastocoel. Enteric cells 

 grow out in two little parenteric masses from the enteron, 

 and then separate widely from one another, spread out, become 

 araoebiform, crawl all over the inner wall of the ectodermic 

 vesicle and line it throughout (see my '^ Observations on Pisi- 

 dium," ' Phil. Trans.,' 1875) , They thus spread themselves out 

 and enclose a large space ; they form ultimately the lining cells 

 of the coelom, so that even where only a few branched cells 

 appear between deron and enteron, they may carry out the 

 essential features of development of the coelom from paren- 

 teric diverticula. 



According to the hypothesis just set forth, we must look, 

 then, m all animals with a coelom, that is to say, in all the 

 higher animals, for parenteric growths, lateral masses of cells 

 of the endoderm, the progeny of which can be traced in 

 further development to the epithelium (the lining cell-mem- 

 brane), of all and any sanguiferous or lymphatic cavities or 

 canals, and to the corpuscles jioating in such cavities. 



The facts observed in the development of higher animals 

 admit very well of this interpretation, only there is this 

 difficulty, that in many cases parenteric growths appear to 

 give rise to a good deal more than the coelom and its epithe- 

 lium. In fact, in Vertebrata the whole of the muscular and 

 skeletal tissues as well, instead of being dclaminated from 

 ectoderm, appear to originate, together with the coelomic 



