PLATE IV. 



Fig. 21.- 



Fig. 21,1. 

 Fig. 22.- 



FiG.22a. 



Fig. 226. 



Fig. 22c. 



Fig. 22d 

 Fig. 22e. 



Fig. 22/ 



Fig. 22^. 



Fig. 23.- 



Fig. 23a. 

 Fig. 24.- 



Fig. 25.- 



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



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



wall, jj Oil Immersion, x 8 Oc. Camera. X 704. 

 — Continuation of above. Taken longitudinally and lateral to that of fig. 21. T V Oil Immersion. X 8 Oc. 



Camera. X 704. 

 -Transverse section through a larva of the same age as the one shown in figs. 21 and 21a. Taken just pos- 

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

 — Continuation of above series. Taken through blastopore. t 't Oil Immersion. X80c. Camera. X 704. 

 — Continuation of above series. Taken just posterior to blastopore. T \ Oil Immersion. X 8 Oc. Camera. 



X 704. 

 — Continuation of above series. Taken halfway between the blastopore and the posterior end. Shows the 



mesoderm cells on the ventral ectoderm. T 'j Oil Immersion. x8 0c. Camera. X 704. 

 — Continuation of above series. Taken through rectum. j*j Oil Immersion. X 8 Oc. Camera. X704. 

 — Continuation of above series. Next section posterior to that of fig. 22rf. T \ s Oil Immersion. X 8 Oc. 



Camera. X 704. 

 — Continuation of above series. 



T 'j Oil Immersion. X 8 Oc. 

 —Continuation of above series 



X704. 

 ■Horizontal section through a larva i if the same age as that of fig. 21 



X704. 

 —Same as fig. 23, but more ventral, ,'j Oil Immersion. X8 Oc. 

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



just made its appearance. r V Oil Immersion. X 8 Oc. Camera. X 704 

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

 122 



Next section posterior to that of fig. 22e, 

 Camera. X 704. 

 Section through nephridial pit. 



Through wall of nephridial pit. 

 ya Oil Immersion. X 8 Oc. Camera. 

 T V Oil Immersion. X 8 Oc. Camera. 

 ( lamera. 



X704. 



Not quite sagittal. 



X225. 



Anus has 



