EARLY STAGES IN DEVELOPMENT OP BALANOGLOSSUS. 225 



Some of the cells lying on the external sides of these masses 

 early become converted into muscular fibres before Stage F is 

 reached (fig. 36, m.f.). A full account of these and the sub- 

 sequent changes occurring in the anatomy of the prseoral lobe 

 willj I hope, be subsequently given, together with a description 

 of the structure of the later stages of the larvse. 



Further Development, Stages F, &c. — With regard to 

 the further development of the animal, I propose on this 

 occasion to speak only of the formation of the mouth, and of 

 the central nervous system. 



The Mouth. — The mouth is found at the end of Stage F 

 as a small perforation in the ventral middle line lying in the 

 groove which separates the prseoral lobe from the collar. At 

 this point the wall of the hypoblast forms a short, downwardly 

 directed diverticulum; this is partially shown in fig. 36. Its 

 outer wall comes into close contact with the epiblast and then 

 fuses with it, a perforation being formed through these 

 coalesced tissues. There is therefore no regular stomodseal 

 invagination. As mentioned above, when first formed the 

 mouth is very minute and quite indistinguishable in a surface 

 view. 



Fig. 37 represents a transverse section through the mouth 

 at a rather later stage. The body cavities occurring on either 

 side of it are portions of the middle pair. 



The Nervous System. — In Stage E a slight thickening 

 of the epiblast could be distinguished in the dorsal middle line 

 of the collar, having a short, longitudinal extent. It may be 

 remembered that in Stage F a slight groove was visible in this 

 position, which existed for a short time and then disappeared 

 (fig. 14, n. g.). 



A series of sections through a larva in which this groove is 

 still visible exhibits the foUoAving arrangement in the epiblast 

 of the collar. In the extreme front portion of the collar the 

 epiblast is slightly thickened in the dorsal middle line, presentiu"- 

 an appearance very like that seen in the same region in Stage E. 

 A depression is also visible on the dorsal surface at the 

 point where the neural groove is cut across. In the middle 



