THEORIES OF THE CCELOM 



19 



fluent with it in the Mollusca and the Arthiopoda. Guided by 

 this erroneous view, I suggested that the reduction of the entero- 

 ccelous pouches of mesobhxst might proceed further than solidifica- 

 tion ; the process of simplification might well be supposed (I 

 suggested) to go on to the reduction of the number of the cells 

 detached from the archenteric M-all, so that eventually a ccelom 



Figs. ;>, 10, and \Ohis. — Three Views of a Young 

 Embryo of the Mollusc Pisidium pusillum. 

 Fig. 9 is viewed from the Surface x'sh shows 

 the Ectodermal (epibla.st) Cells. Fig. 10 

 SHOWS the same Embryo in Optical Median 

 Section, whilst Fig. \Ohis shows a focussing 

 TO a Plane just below the Epiblastic Layer. 



The invaginated archenteric sac (I'ypoblast) hy 

 is seen at one pole. Closely applied to the under 

 surface of the epiblastic layer are numerous branched 

 cells, Hie ; similar cells Qi) appear to be originating 

 by cell-division from the wall of tlie archenteron. 

 The cells rrn: and ;> are "mesenchyme." Possibly 

 among them, near to the archenteric wall, are the 

 mother-cells of the ccelomic pouches. (After Lan- 

 kester, from Balfour.) 



might be formed by a few wandering cells, or even a pair only of 

 such cells, detached from the archenteric wall, and creeping over the 

 ectoderm and endoderm in the space between them which often is 

 enlarged to form a blastoccel. Such cells do occur in Mollusca 

 (Cyclas,^ Lymnaius, Paludina), and probably have to do with the 

 formation of blood-vessels and blood and other skeleto-trophic tissue, 

 though their history has not been traced (see Figs. 9, 10, and Idhis). 



^ See Lankester, " Development of Mollusca," Phil. Trans. 1S73. 



