218 WILLIA5I BATESON. 



taining a cavity which communicates directly with the cavity 

 of the archenteron. Behind these, again, is another pair of 

 regular archenteric diverticula, in the region of the trunk. This 

 mode of origin of the mesoblast, which will be made clear by 

 the diagram (fig. 40), will now be described in detail. It will, 

 however, be perhaps simplest to describe all the remaining 

 partSj beginning at the anterior end of the animal, and pro- 

 ceeding backwards. 



1. The Anterior Body Cavity. 

 On reference to the longitudinal section of the larva between 

 Stages C and D (fig. 25) it is seen that the cells lining the 

 front of the archenteric cavity had a character different from 

 that of the remaining hypoblast. When Stage E is reached 

 this dififerentiation has greatly increased, and the cells which 

 line this anterior region may now be seen to have an entirely 

 peculiar appearance. They are still closely applied to the 

 epiblast (fig. 36, M') at one end. 



This peripheral end is broad, and is continued into a narrower 

 portion, which again dilates to form the round head of the cell, 

 which projects into the cavity of the pra3oral lobe. These round 

 central extremities, in which the nuclei are generally to be 

 found, are continually budding off round cells into the cavity 

 in which they lie. At this period, however, this process of 

 proliferation is only commencing, and the cavity of the prse- 

 oral lobe is therefore lined by a layer of cells, which is for the 

 most part only one cell deep. 



In the peripheral ends of these cells, which, as will be seen 

 hereafter, are destined to form part of the mesoblast, an 

 appearance may be noted which, together with the absence 

 of granules in this part of the cell, gives it a look of semi- 

 fluidity. This appearance is characteristic of a large part of 

 the hypoblastic and mesoblastic tissue in a larva of this age, 

 and disappears about the time at which the mouth is formed. 



Whether it is actually due to the presence of fluid contents 

 in the cells or not I cannot say : since, however, a considerable 

 increase in the size of the body occurs before the animal leaves 



