PLATE IV. 



Fig. 21.- 



Fig. 21a. 



Fig. 22.- 



Fig. 22a. 

 Fig. 226. 



-Sagittal section through a larva somewhat older than that of fig. 20 a, b, c, etc. Blastopore oval and 



transverse. Nephridial pit present at this stage. Archenteron has fused with posterior ectodermal 



wall. T 'j Oil Immersion. X 8 Oc. Camera. X 704. 

 Continuation of above. Taken longitudinally and lateral to that of fig. 21. 



Camera. X 704. 

 -Transverse section through a larva of the same age as the one shown in figs. 



terior to ganglion. ,'.. Oil Immersion. X 8 Oe. Camera. X 704. 

 Continuation of above series. Taken through blastopore, fc Oil Immersion. X80c. Camera. X 704. 



Taken just posterior to blastopore. -, 1 ., Oil Immersion. X 8 Oc. Camera. 



T 'r Oil Immersion. X 8 Oc. 

 II and 21d. Taken just pos- 



Continuation of above series 

 X 7fi4. 

 Fig. 22c. Continuation of above series. Taken halfway between the blastopore and the posterior end. 

 mesoderm cells on the ventral ectoderm. T \, Oil Immersion. X 8 Oc. Camera. X 704. 

 Continuation of above series. Taken through rectum, fa Oil Immersion. X 8 Oc. Camera. 

 Continuation of above series. Next section posterior to that of fig. 22tf. ,', Oil Immersion 



Camera. X 704. 

 Continuation of above series. 

 t 't Oil Immersion. X 8 Oc. 

 Continuation of above series. 



X 704. 

 -Horizontal section tli rough a larva of the same age as that of fig. 21 



X704. 

 Same as fig. 23, but more ventral. T J j Oil Immersion. X 8 Oc. 

 -Longitudinal section through a larva somewhat older than that of tig. 21. 

 just made Its appearance. ,', Oil Immersion. X 8 Oe. Camera. X 704 

 Fig. 25. Larva with two pairs of tentacles. (From life. ) J Ob. X4 0c. Camera. 

 122 



Nest section posterior to that of fig. 22e. 

 Camera. X 704. 

 Section through nephridial pit 



Shows the 



X 704. 

 X 8 Oc. 



Through wall of nephridial pit. 

 r l 2 nil Immersion. X 8 Oc. Camera. 

 T 'j Oil Immersion. X 8 Oc. Camera. 



Camera. 



704. 

 Not quite sagittal. 



X225. 



Anus has 



