1908] Davis. — Life-history of DoJichoglossus. 209 



froDi IJic mrsodrrm. Their walls are one-eell deep, the cells he- 

 coming flattened. For a while the three hody cavities, anterior, 

 middle, and posterior, are continuous, though marked off by the 

 constrictions already described. Fig. 29, pi. 8, shows the relation 

 of these cavities to one another at this stage. Subsequently the 

 constrictions deei)en until finally they completely separate the 

 cavities. 



Prior to this period the middle and posterioi- cavities have 

 extended ventrally somewhat, but more dorsally. This growth 

 continues concomitantly with the later stages of longitudinal 

 growth just described. They extend dorsally, meeting at the 

 mid-dorsal line, but ventrally they do not quite meet. 



As the above account of the origin of the middle and posterior 

 body cavities in D. piisillus is at absolute variance with Bateson's 

 description of theorigin of these structures in B. kowalevskii, and 

 with his diagram, familiar in text-books, I wish to review his 

 account somewhat fully. 



After describing the stage corresponding to that in D. i)i(siUus 

 shown in fig. 23, pi. 8, he says: "The mesoblast arises at this 

 period of development. It is formed directly by differentiation of 

 cells belonging to the archenteron. These differentiations occur 

 in five regions. The first comprises a median and primitively- 

 unpaired tract in the anterior end, which forms the lining of the 

 body cavity of the praeoral lobe. Behind this anterior body 

 cavity a pair of mesoblastlc differentiations occur in the region 

 of the colhir, constituting lateral outgrowths of the archenteric 

 walls, each containing a cavity which communicates directly with 

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

 l)air of regular archenteric diveiticula, in the region of the 

 trunk." He refers at this point to his well known diagram 

 illustrating his description. 



, He then describes in detail (pp. 141-142) the development of 

 the anterior body cavity. The latter part of his description (pp. 

 218-219) is as follows: 



"Now, since the anterior body cavity is continued behind the 

 end of the gut on all sides excepting the ventral, it is creseentic 

 in shape, the concavity being dii'ccted downwards. This appear- 

 ance exists only for a short distance. Behind it the continuity 



