210 University of California Puhlications in Zoology. [Vol. 4 



across the dorsal surface ceases, and the mesoblast exists a^ a pair 

 of small, hollow cavities at the dorso-lateral sides of the gut, which 

 is here much more fully developed, occupying most of the space 

 enclosed by the epiblast. Still farther backward the cavities in 

 these two mesoblastic tracts close up, and their walls are continued 

 for a short distance as two solid cords of cells, and then disappear. 



"The mesoblast of the anterior body cavity is, therefore, 

 formed directly from the walls of the hypoblast, which occupied 

 the same situation. It is separated off from it by a process of 

 constriction in the region of the external groove, dividing the 

 proboscis from the collar. While this process of constriction is 

 being carried out, tJie pouch of mesoblast grows backwards, sur- 

 rounding the gut except on the ventral surface, but especially 

 forniiug the liollow ttoriis.'' (Italics mine.) 



The description which I have just quoted agrees substantially 

 with my own observations of the origin and first stages of devel- 

 opment of the mesoblast in its relation to the anterior body cavity. 

 The portions of his description of the lateral backward growths 

 of the mesoblast which I have italicized is almost an exact account 

 of the stage which I have shown in fig. 26, and I believe must 

 correspond to it. As I have shown, the mesoderm already grow- 

 ing backwards continues to do so at the sides, finally forming the 

 middle and posterior body cavities. This conclusion is based on 

 an examination of a large number of sections (about one hundred 

 and fifty series of sections), a typical series of which I have 

 shown in my figures. Nowh.ere have I seen any indication of a 

 communication between any one of tlie middle or posterior cavities 

 with the enteric canal. Neither have I seen any evidence of de- 

 lamination of the enteric wall. Throughout all the stages, from 

 the two-walled cylinder to that of the fully formed body cavities, 

 the walls of the enteron in the region of the middle and posterior 

 body cavities show no histological difference from other parts of 

 the wall. Whereas the cells bounding these cavities are very 

 different from the cells making up the wall of the enteron. 



Concerning the connection between the middle body cavities 

 and the archenteron Bateson '84 (p. 220) says: "This condition 

 is onlv visible in a very few of the larvae, and may possibly be 



