DEVELOPMENT OF FLUSTRELLA HISPIDA. 477 



' Fig, 54 b. — Transverse section of the larva at the twenty-cell stage, sliow- 

 ing the formation of nuclear spindles in the aboral cells h.\ — C^, and in the 

 oral cell k\, prior to division to form cells of the sixth generation. X 225. 



Pig 55 a, b. — Transverse sections of the larva at the twenty-cell stage, but 

 at a slightly later period than that illustrated in Figs. 54 ff, b, the division of 

 two of the aboral cells being now completed, x 400. 



Fig. 5G. — Longitudinal section of the larva at the thirty-two cell stage. 

 X 225. 



Fig. 57. — Longitudinal section of the larva showing the formation of the 

 primitive mesendoderm cells by the division of the four large central oral cells. 

 X 225. 



Fig. 58. — Section of the larva showing the enclosure of the four large 

 central oral cells within the primitive segmentation cavity, forming, together 

 with the four cells previously enclosed, in all eight mesendoderm cells ; a 

 blastopore (^Bl.) is represented, x 225. 



Fig. 59. — Section of the larva at a slightly later stage than that shown in 

 the previous figure. The difference in size between the cells of the aboral and 

 oral ectoderm is noticeable ; the eight primitive mesendoderm cells have 

 divided, forming a mesendoderniic mass which projects from the oral face into 

 the segmentation cavity ; the blastopore is still open, x 225. 



Fig. 60. — Section of the larva at a stage subsequent to the closure of the 

 blastopore. M.C. marks the position of a slight groove which is to be seen 

 on the exterior of the larva, and the slight protuberance visible below this in 

 the external view is due to the difference in size of the cells of the oral and 

 aboral ectoderm, which meet along this line. The mesendoderm almost com- 

 pletely fills the segmentation cavity, x 225. 



PLATE 25. 



Fig. 61. — Section of an older larva in which the formation of the mesen- 

 doderm is completed, but in which the larval organs have not yet begun to 

 form. The difference in size between the cells of the oral and aboral ectoderm 

 is now very marked ; the groove M.C. has deepened, and the oral cells below 

 it are slightly protruded, giving rise to a raised ring on the external surface 

 of the larva ; this ring is, however, quite unconnected with the ciliated ring 

 which develops later. The mesendodermic mass has by this time completely 

 obliterated the segmentation cavity, x 225. 



Fig. 62. — Longitudinal section of a young larva at the period when the 

 larval organs are beginning to form. Tliree oral invaginations are observable; 

 these will later develop into the internal sac, pharynx, and pyriform organ, 

 respectively. Globules (y.) are seen to be exuding from the cells of the internal 

 sac, and similar globules are also present within the cells of the internal sac 



