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- 

 ccelous 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. 0, 10, and lObis. THREK VIEWS OF A YOUNG 

 EMBRYO OF THE MOLLUSC PISIDIUM PUSILLUM. 

 FlG. IS VIEWED FROM THE SURFACE AND SHOWS 



THE ECTODERMAL (epiblast) CELLS. FlG. 10 

 SHOWS THE SAME EMBRYO IN OlTlCAL MEDIAN 



.SECTION, WHILST FIG. IQbis SHOWS A FOCUSSING 

 TO A PLANE JUST BELOW THE EPIBLASTIC LAYER. 



The invayinated archenteric sac (hypoblast).//// 

 is seen at one pole. Closely applied to the under 

 surface of the epiblastic layer are numerous branched 

 cells, me ; N similar cells d>) appear to be originating 

 by cell-division from the wall of the archenteron. 

 The cells we and p are " mesenchyme." Possibly 

 among them, near to the archenteric wall, are the 

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

 kester, from Balfour.) 



B. 



FIG. 10. 



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 Lymnaeus, 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. 



