EAELY STAGES IN DEVELOPMENT OF BALANOGLOSSCJS. 235 



Stages C and D. The letter H is placed in the archeuteric cavity. (Obj. D, 

 oc. 2.) 



Tig, 24. — Transverse section through posterior end of the same larva as the 

 preceding. The transverse band of cilia and the point at which the blasto- 

 pore has closed are cut through. (Obj. C, oc. 2.) 



Fig. 25. — Longitudinal median vertical section through same larva as pre- 

 ceding. Constructed from the series of transverse sections, in which those 

 represented in Figs. 23 and 24 occur. 



Figs. 26 — 34 represent some of a series of transverse sections taken through 

 a larva in Stage E. The epiblast is represented diagrammatically. In each 

 case the upper side is the dorsal one. (All, excepting Fig. Zla, were drawn 

 with Obj. D, oc. 2.) 



Fig. 26. — Section through prseoral lobe, showing the amoeboid cells forming 

 the wall of the mesoblastic sac. 



Fig. 27. — Section through the foramen, by which the anterior body cavity 

 opens into the archenteron. The space lying ventral to the hypoblast is the 

 slight ventral backward prolongation of the anterior body cavity, which would 

 diaappear in the next few sections. 



Fig. 28. — Through the ends of the posterior horns of the anterior body 

 cavity, whose cavities are not prolonged to this point. In this section the 

 rudiment of the nervous system is shown. 



Fig. 29. — Section between the anterior and middle mesoblastic regions. 



Fig. 30. — Section through the junction of the second body cavities and the 

 archenteric cavity. 



Fig. 31.— Section through the middle of the second body cavities. 



Fig. 31«. — Part of a section similar to the preceding. It represents the 

 minute structure of the epiblast and hypoblast, and of the mesoblastic cells 

 lining the second body cavity, which are here still connected with the hypo- 

 blast. (Obj. F, oc. 2.) 



Fig. 32. — Section through the region between the second and third meso- 

 blastic tracts. 



Fig. 33. — Section through the anterior region of the third mesoblastic 

 tracts. 



Fig. 34. — Section taken rather behind the preceding, traversing the 

 foramina by which the posterior body cavities open to the archenteron. 



Fig. 35. — A longitudinal median section of a larva between Stages D and 

 E, showing the opening of the anterior body cavity into the archenteron. 

 (Obj. C, oc. 2.) 



Fig. 36. — Longitudinal nearly median section, almost in the vertical plane, 

 through a larva between Stages E and F. (Obj. C C, oc. 2.) The anterior 

 body cavity is now seen to be closed. 



Fig. 37. — Transverse section through the mouth of a larva between Stages 

 F and G. (Obj. D, oc. 2.) 



