404 ARTHDR T. MASTERMAN. 



Tornaria has been carefully followed by Morgan (14a). 

 He finds that there is " a process of proliferation of the walls 

 of the stomach, so that the wall at this point, by division 

 of its cells, becomes two-layered." The mesoblastic mass, 

 thus derived, has at first no cavity. The whole process 

 is precisely similar to that of Ph or on is, as described 

 above. 



In the demersal larva Bateson states that the collar meso- 

 blast arises by a modified form of archenteric diverticula. 

 " This pair of cavities is bounded on the inner sides by the 

 cells forming the wall of the gut, and the external boundary 

 is made up of a single layer of cells continuous dorsally 

 and ventrally with the hypoblast." As a matter of fact, 

 Bateson's reasons for regarding them as archenteric diver- 

 ticula appear to be two. Firstly, there is a connection of 

 their cavity with that of the archenteron which occurs in 

 " very few larvee ;" secondly, he notes that " the middle 

 mesoblastic tracts in Tornaria are said to be archenteric 

 diverticula." Under any circumstances the inner walls 

 would appear to arise from the hypoblast in a similar 

 manner to the mesocoele in Tornaria, according to Morgan 

 (14a), and the mode of formation is really a type inter- 

 mediate between the formation of archenteric diverticula 

 and delamination. Bateson points out that these meso- 

 blastic pouches extend principally posterior to their point of 

 origin, which is also the case in Phorouis. 



4. The third pair or trunk somites arise in Tornaria 

 (Morgan) as a pair of hypoblastic outgrowths, two- walled 

 from the first, but containing no cavity till after separation 

 from the hypoblast. In the demersal larva they arise as a 

 pair of archenteric diverticula. In Phoronis I am unable 

 to state with certainty that they arise either as hypoblastic 

 outgrowths actually involving the wall of the archenteron, or 

 whether they are formed by active division from the distal 

 ends of archenteric cells, but very probably by one of these 

 two methods. In this respect Phoronis resembles Tor- 

 naria rather than the demersal type. Bateson (la) states 



