PLATE III. 



Fig. 17'/. Same as fig. 17, showing one of the peculiar cells set free in the blastoccele. fa Oil Immersion. X 8 Oc. 



Camera. X 704. 

 Fin. 18. Transverse section through the anterior end of a young larva, which is slightly younger than the specimen 



nf which fig. 19 is a longitudinal section. Blastopore is slightly oval in outline. Taken through the 



ganglion, /.oil Immersion, x 8 Oc. Camera. 704. 

 Fig. 18a. Continuation of above series. Taken just in front of archenteron. fa Oil Immersion. 80c. Camera. 



X 7(14. 

 Fig. 186. Continuation of above series. Taken through anterior part of blastopore, fa Oil Immersion. X 8 Oc. 



Camera. 704 

 Fig. I8e. Continuation of above series. Taken through posterior part of blastopore, fas Oil Immersion. X 8 Oc. 



Camera. 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." faOU Immersion. 



X 8 Oc. Camera. X 704. 

 Fig. 18e. Continuation of above series. Taken posteriorly to section 18d. No sign of Caldwell's " primitive groove 



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

 Fu;. 18/. Horizontal section through same stage as 18 a, b, c, etc. fa i lil Immersion. X 8 Oc. Camera. X 704. 

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



preoral sac. T V Oil Immersion. X 8 Oc. Camera. 7(i4. 

 Fig. 20. Sagittal section through a larva somewhat older than that of fig. 19. Blastopore has become circular again, 



but much smaller than originally, fa- Oil Immersion. X 8 Or. Camera. X 704. 

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



fa Oil Immersion: 8 0c. Camera. X 704. 

 Fig. 206. Continuation of series. Taken through blastopore. fa oil Immersion. 8 0c. Camera. X 704. 



Fig. 20 < ontinuation of series. Taken just posterior to the blastopore. .Slight indication of groove, fa Oil 



Immersion. 8 Oc. Camera. X 704. 

 Fig. 20d. Continuation of series. Taken near posterior end of larva. No groove, fa Oil Immersion. X 8 Oc. 



Camera. 7m. 

 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, fa Oil Immersion. X 8 Oc. Camera. X 704. 

 120 



