224 WILLIAM BATESON. 



importance consist in the closing oflf of these five mesoblastic 

 pouches. The foramina opening from the archenteron to the 

 middle pair are the first to become obliterated by the coalescence 

 of the hypoblast surrounding them. Shortly after this has 

 occurred the layers of cells forming their inner wall segregate 

 themselves from the archenteric wall. 



This process leads to the formation of a pair of closed sacs 

 lying one on each side of the archenteron in the region of the 

 collar. 



While the middle pair are separating, the process of closure 

 is also completed between the anterior body cavity and the 

 archenteron by the fusion of the margins of the hypoblastic 

 tube previously described as projecting into the anterior body 

 cavity. The cells composing this tube then retreat backwards, 

 and cease to project beyond the limit of the general anterior 

 wall of the gut. The resulting condition is shown in fig. 36, 

 which is drawn from a longitudinal section which does not pass 

 quite through the middle dorsal line, and is nearly parallel to 

 the longitudinal vertical plane of the body. (Owing to this fact 

 the nervous system is not shown.) 



In this section the cells forming the posterior wall of the 

 anterior body cavity form a layer in apposition wuth the outer 

 ends of the cells constituting the anterior wall of the gut in the 

 region at which the communication between the two cavities 

 previously existed (vide fig. 35). The anterior wall of the gut 

 is here composed by a single layer of cells. 



The posterior mesoblastic pouches are shut off from the 

 archenteron in a manner similar to that of the middle pair. 



On the completion of the separation of these various portions 

 of mesoblast from the gut the cells of their walls change their 

 primitive character. 



Subdividing rapidly they become much smaller, assuming, 

 except in the case of those of the anterior body cavity, the 

 appearance of ordinary peritoneal cells. Those of the anterior 

 body cavity proliferate rapidly in the manner mentioned above, 

 especially in the dorsal and ventral middle lines, forming a 

 pair of large masses of spherical cells in these situations* 



