THE ENTEROPNEUSTA. 65 



Fig. 110. — Transverse section of young ovary of B. Kowalevskii (young 

 adult). Obj. D, oc. 2. 



Fig. 111. — Group of ovarian follicles of B. Kowalevskii, older than the 

 above. Obj. D, oc. 2. 



Fig. 112.— Ripe ovarian follicle of B. Kowalevskii. Obj. A, oc. 2. 



PLATE XII. 



Diagrams. — Skin coloured light blue; nervous system, dark blue; 

 hypoblast, light red; blood-vessels, dark red; mesoblast, green. 



Fig. 1. — Blastosphere. 



Fig. 2.— Gastrula. 



Fig. 3. — Longitudinal vertical section througli gastrula; blastopore being 

 nearly closed. 



Fig. 4. — Ditto ; blastopore closed. 



Fig. 5. — Ditto, later stage; mesoblast forming. 



Fig. 6. — Longitudinal horizontal section through a somewhat later stage. 



Fig. 7. — Longitudinal horizontal section through larva in Stage G. 



Fig. 8. — Transverse section of collar of foregoing in plane of line d d. 



Fig. 9. — Longitudinal vertical section of Stage H. 



Fig. 10. — Transverse section of collar of foregoing in plane of line d d. 



Fig. 11. — Longitudinal horizontal section of adult in plane of heart.. (This 

 plane would not really take in the perihaemal cavities, but their relations are 

 tlius made clear.) 



Fig. 12. — Transverse section of foregoing in plane of line d d. 



Fig. 13. — Longitudinal horizontal section of junction of collar and trunk in 

 a larva of about 4 gill-slits. 



Fig. 14. — Similar section to foregoing of adult, showing formation of oper- 

 culum, atrial cavity, and collar funnels. 



