CCELENTERATA. 



x-m 



b-- 



FIG. 70. DIAGRAMMATIC 

 FIGDHE SHEWING THE DELA- 

 MINATION OF THE OVUM OP 

 GERYONIA. (Copied from 

 Fol.) 



cs. segmentation cavity; 

 a. endoplasm; 6. ectoplasm. 

 The dotted lines shew the 

 course of the next planes of 

 division. 



central mass of a more spongy character. The segmentation is com- 

 plete and regular, and up to the time when thirty-two segments have 

 appeared each segment is composed of both 

 constituents of the protoplasm of the ovum. 

 A segmentation cavity appears when sixteen 

 segments are formed, and becomes somewhat 

 larger at the stage with thirty-two. At this 

 stage the process of delamination commences. 

 Each of the thirty-two segments, as shewn in 

 the accompanying diagram (fig. 70), becomes 

 divided into two unequal parts. The smaller 

 of these is formed almost entirely of granular 

 material ; the larger contains portions of 

 both kinds of protoplasm. In the next seg- 

 mentation the thirty-two large cells only are 

 concerned, and in each of these the line of 

 division passes between the granular and the 

 transparent protoplasm. The sixty-four len- 

 ticular masses of granular protoplasm thus 

 formed constitute an outer closed epiblastic 

 vesicle, within which the thirty-two masses of transparent protoplasm 

 form an hypoblastic vesicle. The embryo at this stage is shewn in 

 optical section in fig. 71. 



The epiblastic vesicle now grows rapidly, 

 while the hypoblastic vesicle remains nearly 

 passive and becomes somewhat lens-shaped. 

 At one point its wall comes in close contact 

 with the epiblast. Elsewhere a wide cavity 

 is developed between the two vesicles which 

 becomes filled with gelatinous tissue. At this 

 period cilia appear on the surface, and the 

 larva becomes a planula. 



The succeeding changes lead rapidly to 

 the formation of a typical Medusa. Where 

 the epiblast and hypoblast are in contact the 

 former layer becomes thickened and forms a disc-shaped structure. 

 The centre of this becomes somewhat protuberant, fuses with the 

 hypoblast and then becomes perforated to form the mouth (fig. 72 o). 

 The edge of the disc forms a thickened ridge, the rudiment of the 

 velum (v), which is entirely formed of epiblast. At its edge six ten- 

 tacles (t) arise, into which are continued solid prolongations of the wall 

 of the now somewhat hexagonal gastric chamber. The hypoblastic 

 axes of the tentacles soon lose their connection with the gastric 

 wall. 



Up to this time the larva has retained a more or less spherical 

 form, and the cavity on the under side of the umbrella has not yet 

 become developed. The latter now becomes established by the whole 

 disc assuming a vaulted form with the concavity directed downwards. 



FIG. 71. EMBRYO OF 

 GERYONIA AFTER DELAMINA- 

 TION. (After Fol.) 



ep. epiblast; luj. hypo- 

 blast. 



B. E. 



9 



