THE CTENOPHORA 



II 



wards. As a result of this change of position a central cavity, 

 the enteron, is formed, aTid the mesoblast cells are carried 



' iz. 



Fio. IV. — Development of Callianira hialata (after Metsclinikoff). 



1. — Ovum surrounded by the vitelline membrane. 



2. — Stage with eight blastomeres. 



3. — Side view of a stage with sixteen blastomeres, eight larger macromeres, mac, and eight 

 smaller niicromeres, mic. 



4. — A similar stage viewed from above. 



5. — Side view of a later stage ; the niicromeres have increased in number, and the macro- 

 meres are beginning to divide. 



6. — Aborai surface of an older embrj'o. The niicromeres form a four-rayed jilate, covering 

 the upper surfaces of the macromeres and surrounding a cross-shaped cavity, the pseudo- 

 blastopore, phi. 



V. — Vertical section of the same embryo as the preceding, .showing the large macromeres 

 covered by the micromeres, except in the regions of blastoi)ore, W, and pseudoblastopore, psb. 

 The nuclei of the macromeres are now at the blastoporic pole. 



8.— Oral surface of an older embryo, hi, blastopore ; mes, mesoblast plate ; ec, ectoderm ; 

 en, endoderm. 



9. — Vertical section of an older embyro showing invagination, vtes, mesoblast ; the pseudo- 

 blastopore is closed. 



10. — An embryo somewhat older than 0. 



11. — A later stage showing the stomodaeum, st, the enteron, eyxt, and the mesoblast, vies, 

 which is spreading out as a plate on either side beneath ectodermic thickenings, which are the 

 priniordia of the tentacles. 



12, — Aborai view of a somewhat later stage, showing the cross-shaped mesoblast plate. 

 WW, wandering cells of the mesoblast. 



13. — Vertical section in the transverse plane of an embrj'O in the same stage as 12. so, .sense 

 organ ; mes, mesoblast ; tr, primordium of the tentacle ; ent, enteron ; st, stomodaeum. 



14.— A later stage, tt, tentacles ; mt, contractile mu.scles of the tentacles fonned from the 

 mesoblast ; ms, mescnchymatous cells derived from, the wandering cells of the mesoblast 

 shown in 12. 



upwards from the blastoporic towards the pseudoblastoporic pole 

 (9). According to Riches, the pseudoblastopore is closed before 



