ox DEVELOPMENT ETC. OF PIIOROXIS. 023 



against tlio tip of tlio divorticnlniii. Xo (lonl)t tlio mf'so])last is 

 hero arising, not l)y dii-ect coll multiplication, but l)y the ])usliiug 

 in of tlio colls of tlio clivorticuluni. This is more clearly illustrated 

 in fig. 31, "which shows a transverse section through the lilasto- 

 pore of a more advanced larva ; hero the mesol)last cells almost 

 fill uyi the blastocodic cavity on both sides of the blastopore. 

 In fig. 30 b, a transverse section just l)ehind the closure of the 

 blastopore, the most anterior portion of the primitive groove before 

 montionod is cut across. Here the wall of the o-roove underlvinçr 

 the gut is formed of uiutually compressed cells, some of which are 

 evidently migi'ating into the blastocœle (on the left-hand side in 

 the figure). If the sections are followed further posteriorly, the 

 groove still persists, but no mesoblast cell in the actual immigrating 

 process can be discovered, although there are those which have 

 been previously pushed out and are now floating between the two 

 primary germinal layoi's at this region. Still more posteriorlv the 

 groove entirely disaj^pears and the entoblastic and ectoblastic 

 layers are separated from each other by the com|)ai'atively wide 

 blastocœlic cavity (fig. 30 r). At this stage, therefore, the greater 

 p'.rt of the archenteric wall has ceased to conti-il)ute towards the 

 mesoblast-formation ; in other words, it has lost its mesentoblastic 

 nature. The mesoblast is now being produced only from two 

 limited regions, viz., anterior divei'ticula and the ventral groove. 

 In a slightly more advanced larva, the ventral groove is still 

 present for some distance immediately behind the blastopore, but 

 the layer wliich forms the groove has ontirelv ceased to give rise 

 to mesoblast colls (fig. 32, which is taken from a transverse sec- 

 tion very near the l)lastopore). It ap]K'ars to me that this groove 

 is to be regarded as Init the posterior portion of the original 

 mesentoblast, wdiich, owing to the flict that the central depression 



