PLATE III. 



Fig. 17'/. — Same as tig. 17, showing one of the peculiar cells set free in the blastocoele. ,'.. Oil Immersion. X 8 Oc. 



Camera. X 704. 

 Fig. IS. — Transverse section through the anterior end of a young larva, which is slightly younger than the specimen 



of which tig. lit is ,\ longitudinal section. Blastopore is slightly oval in outline. Taken through the 



ganglion. T V Oil Immersion. X 8 Oc. Camera. X 704. 

 Fig. ISc;. — Continuation of above series. Taken just in front of archenteron. ,\ Oil Immersion, x 8 0c. Camera. 



X 704. 

 Fig. 186. — Continuation of above series. Taken through anterior part of blastopore. T l , Oil Immersion. X 8 I »c. 



Camera. X 704. 

 Fig. 18c. — Continuation of above series. Taken through posterior part of blastopore. j 5 Oil Immersion. X 8 Oc. 



Camera. X 704. 

 Fig. 18d. — Continuation of above series. Taken immediately posterior to the blastopore. Lips have grown together 



and there is a straight furrow corresponding to Caldwell's "primitive groove." rj Oil Immersion. 



X 8 Oc. Camera. X 704. 

 Fig. 18c. — Continuation of above series. Taken posteriorly to section 1S</. No sign of Caldwell's "primitive groove 



or streak." -fa Oil Immersion. X 8 Oc. Camera. X 704. 

 Fig. 18/. — Horizontal section through same stage as 18 a, b, c, etc. fa Oil Immersion. X 8 0c. Camera. X 704. 

 Fig. 19. — Sagittal section through larva which is a little older than that of series 18 a, 6, c, etc. Shows mesodermal 



preoral sac. jV Oil Immersion. X 8 Oc. Camera. X 704. 

 Fio. 20. — Sagittal section through a larva somewhat older than that of tig. 19. Blastopore has become circular again, 



but much smaller than originally, T V Oil Immersion. X S Oc. Camera. X 704. 

 Fig. 20a. — Transverse section through larva of same age as that of fig. 20. Taken just posterior to the ganglion. 



T V Oil Immersion. X S ( >c. Camera. < 704. 

 Fig. 206. — Continuation of series. Taken through blastopore. T ' 5 Oil Immersion. X 8 Oc. Camera. X 704. 

 Fig. 20c. — Continuation of series. Taken just posterior to the blastopore. Slight indication of groove. T 'o Oil 



Immersion. X 8 Oc. Camera. X 704. 

 Fig. 20rt\ — Continuation of series. Taken near posterior end of larva. Xo groove. T ' 5 Oil Immersion. X 8 Oc. 



Camera. X 704. 

 Fig. 20e. — Horizontal section through a larva of the same age as that of figs. 20 a, 6, etc. Shows mesodermal sac 



with posterior prolongations. ,\, Oil Immersion. X 8 Oc. Camera. X 704. 

 120 



