ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 387 
PLATE XXNI. [PLATE 30]. 
Figs. 1, 2,3, & 4. Ovisacs containing complete ova in different stages of development. The 
fraction above each figure gives the greatest diameter in parts of an inch. 
Fig. 5. An ovisac, tornat one point, but otherwise entire, and allowing the naked germinal 
vesicle to be seen through its wall. 
Fig. 6. A similar ovisac opened with needles, and the torn lower portion, to which the 
germinal vesicle adheres, viewed from within. 
Fig. 6 a. An enlarged view of the germinal vesicle. 
Fig. 6 4. A similarly magnified view of the dilated end of the duct of the same specimen, 
showing the ‘ plug’ of spermatozoa. 
Fig. 7. A more advanced ovisac opened and viewed from within, 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. 
Figs. 8-8c. Germinal vesicles containing the characteristic deposit o. more advanced 
stages. 
Fig. 8 @ A germinal vesicle with a very pale spot. Its contour is rather too well defined in 
the figure. 
Fig. 8 46. Two views of the same germinal vesicle, whose spot has disappeared. The min- 
ute vesicular corpuscles, ev, are visible. 
Fig. 8c. Front and lateral views of a germinal vesicle in a condition observed once. 
Fig. 9. Ovisac with vesicular epithelial lining and commencing blastoderm (which is 
represented rather darker than in nature) 27 sz¢ and causing the atrial tunic, d, to 
bulge as a rudimentary ‘ Brutsack’ or foetal chamber. 
g. 9. a. The blastoderm of a similar specimen enlarged, and viewed through the ¢zzca 
propria of the ovisac. To avoid confusion, the texture of the vesicular lining is 
omitted. 
N.B. The figures 8, 8*, 8a, 34, 8c, 9 a, are drawn to the same scale. 
Fig. 10. Foetus, 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. i 
Fig. 12. The fourth ascidiozooid of a similar icetus, 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. 13a. 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 
Fi 
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the ovisac. 
g. 15. One of the most advanced foetuses observed. The remains of the conjoined cyatho- 
zooid and ovisac are 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 4th and ;'sth of an inch in dia- 
meter respectively. 
Fig. 19. Lateral view of a foetus Ast of an inch in diameter, to show the manner in which 
the cloaca is developed by the separation of the test from the combined cyatho- 
zooid and ovisac. 
Fi 
a 
