218 



LA MELLIBRA XCHIA TA . 



now develops which forms the commencement of the foot, and the archen- 

 teric sack grows out at its free extremity into two lobes, but remains 



A. 



/ 'H^->" " . i'.^4.?.* If^V"? 



B. 



FIG. 119. THREE VIEWS OF AN EMBRYO OF PISIDIUM IMMEDIATELY AFTER THE 

 CLOSURE OF THE BLASTOPORE. (After Lankester. ) 



A. View from the surface. 



B. Optical section through the mediau plane. 



C. Optical section through a plane a little below the surface. 



ep. epiblast; me. mesoblast; hy. hypoblast; p. cells apparently budding from the 

 hypoblast to form rnesoblastic elements. 



attached to the epiblast by an im perforate pedicle. The next organ to appear 

 is the stomodseum. It arises as a ciliated epiblastic ingrowth which meets 

 the free end of the archenteric sack, fuses with it, and shortly after- 

 wards opens into it (fig. 118, ph). Between the mouth and the at- 

 tachment of the enteric pedicle is placed the foot (f), which becomes 

 ciliated. On the dorsal side of the enteric pedicle there appears a saddle- 

 shaped patch of epiblast cells bounding the sides of a groove (shs). This 

 is the rudiment of the shell-gland. 



The enteric pedicle, or intestine as it may now be called, soon 

 acquires a lumen, though still imperforate at its termination where the 

 anus is eventually formed. Ventral to the intestine is placed a mass of 

 cells the rudiment of the organ of Bojanus. It is stated to be de- 

 veloped as an ingrowth of the epiblast. 



