THEORIES OF THE CCELOM 



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

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

 coelous pouches of mesoblast 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 wall, so that eventually a ccelom 



FIGS. 9, 10, and lObis. THREE VIEWS OF A YOUNG 

 EMBRYO OF THE MOLLUSC PISIDIUM PUSILLUM. 

 PlQ. 9 IS VIEWED FROM THE SURFACE AND SHOWS 

 THE ECTODERMAL (epiblast) CELLS. FIG. 10 



SHOWS THE SAME EMBRYO IN OPTICAL MEDIAN 



SECTION, WHILST FIG. lObis SHOWS A FOCUSSING 

 TO A PLANE JUST BELOW THE EPIBLASTIC LAYER. 



The invaginated archenteric sac (hypoblast).% 

 is seen at one pole. Closely applied to the under 

 surface of the epiblastic layer are numerous branched 

 cells, me ; similar cells (p) appear to be originating 

 by cell-division from the wall of the archenteron. 

 The cells me and p are " mesenchyme." Possibly 

 among them, near to the archenteric wall, are the 

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

 kester, from Balfour.) 



KIG. 0. 



C. 



FIG. lO&ts. 



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, 1 Lymnzeus, 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 IQbis). 



1 See Lankester, " Development of Mollusca," Phil. Trans. 1873. 



