250 PROFESSOR HUXLEY ON THE ANATOMY, ETC., OF PYROSOMA. 



Fig. 7. A more advanced ovisac opened and viewed from witiiin, showing the pale germinal vesicle covered 



by the epithelial coating of the sac. 

 Fig. 8*. An ovum extracted from a younger ovisac than the last. 



Flo's. 8-Sc. Germinal vesicles containing the characteristic deposit of more advanced stages. 

 Fig. 8 a. A germinal vesicle with a very pale spot. Its contour is rather too well defined in the figure. 

 Fiw, 8 b. Two views of the same germinal vesicle, whose spot has disappeared. The minute vesicular cor- 

 puscles, en, are visible. 

 FitT. 8 c. Front and lateral views of a germinal vesicle in a condition observed once. 

 Fig. 9. Ovisac with vesicular epithelial hning and commencing blastoderm (which is represented rather 



darker than in nature) in situ and causing the atrial tunic, d, to bulge as a rudimentary 



' Brutsack ' or foetal chamber. 

 Fig. 9 a. The blastoderm of a similar specimen enlarged, and viewed through the tunica propria of the 



ovisac. To avoid confusion, the texture of the vesicular lining is omitted. 

 N.B. The figures 8, 8*, 8a, 8b, Sc, 9«, are drawn to the same scale. 

 Fig. 10. FcEtus, now free in the mid-atrium, with the blastoderm much enlarged and converted into an 



elongated patch. 

 Fig. 11. Two views of a foetus with the blastoderm divided into five segments, of which the cyathozooid 



is the largest. 

 Fig. 12. The fourth ascidiozooid of a similar foetus, seen in section, and the fifth from above. 

 Fig, 13. Two views of a foetus whose ascidiozooids half encircle the base of its cyathozooid. 

 Fig. 13 a. A single ascidiozooid (the first) of a similar foetus, seen from the side. 

 Fig. 14. A more advanced foetus, to show the stage in which the ascidiozooids (left in outline) completely 



encircle the cyathozooid, but still lie below the level of the equator of the ovisac. 

 Fig. 15. One of the most advanced foetuses observed. The remains of the conjoined cyathozooid and 



ovisac ai-e hidden by the ascidiozooids. 

 Fig. 16. A similar foetus viewed from above, to show the remains of the cyathozooid and the ovisac, 



as well as the connexion of this with the ascidiozooids, and of these with one another, by the 



elongated isthmuses. 

 Figs. 17 & 18. Lateral views of the cyathozooid in foetuses J^th and J^^th of an inch in diameter respect- 

 ively. 

 Fig. 19. Lateral view of a foetus ^st of an inch in diameter, to show the manner in which the cloaca is 



developed by the separation of the test from the combined cyathozooid and ovisac. 



