530 McMURRICH. 



EXPLANATION OF PLATE XIII. 



Figs. 1-12 are oi Metridium marginatum. 



Fig. I. Immature ovum taken from the ovary. 



Fig. 2. Commencement of the first cleavage. 



Fig. 3. First cleavage about half completed. 



Fig. 4. Flattening out of spherules at conclusion of the first cleavage. 



Fig. 5. Four-celled stage. 



Fig. 6. Eight-celled stage. 



Fig. 7. Early blastula; figure drawn from preserved ovum. 



Fig. 8. Optical section of free-swimming blastula. 



Fig. 9. Optical section of blastula, just before the breaking through of the mouth, 

 showing the depression at the posterior pole. Drawing made from preserved speci- 

 men. 



Fig. 10. Optical section of gastrula stage ; drawn from a preserved and cleared 

 specimen. 



Fig. II. Section through blastula, 48 hours old. Camera. Zeiss, D, 2. 



Fig. 12. Section through embryo, showing the formation of endoderm by delami- 

 nation. Camera. Zeiss, D, 2. 



Figs. 13-18 are from Rhodactis Sajtcti-ThomcE. 



Fig. 13. Portion of longitudinal section through anterior (aboral) pole of young 

 larva. <r« = endoderm, w^= mesogloea, « = nerve-layer, f<r = ectoderm. Camera. 

 Zeiss, J, 2. 



Fig. 14. Transverse section through larva, showing formation of second mesentery. 

 Camera. Zeiss, A, 4. 



Fig. 15. Transverse section through larva, showing formation of second mesentery 

 by apposition of stomatodaeum to its line of formation. Camera. Zeiss, A, 4. The 

 details of the endoderm in this and the three succeeding figures are not completed. 



Fig. 16. Transverse section, showing the formation of the third and fourth pairs 

 of mesenteries. Camera. Zeiss, A, 4. 



Fig. 17. Transverse section through larva, showing reflection of stomatodseal ecto- 

 derm. Camera. Zeiss, A, 4. 



Fig. 18. Transverse section through larva, showing ectoderm on outer surface of 

 stomatodaeum, apparently having broken through the mesoglcea. Camera. Zeiss, A, 4, 



Figs. 19-24 are from embryos oi Aulactinia stelloides. 



Fig. 19. Transverse section of larva, showing the formation of the fifth and sixth 

 pairs of mesenteries. Camera. Zeiss, A, 2. 



Fig. 20. Transverse section of mesenterial filament of the third pair of mesen- 

 teries of larva with only four pairs of perfect mesenteries. Camera. Zeiss, D, 2. 



Fig. 21. Transverse section through a mesentery of the fifth pair, just below the 

 point where it separates from the stomatodaeum. 5/=stomatodffial ectoderm, en' — 

 endoderm of mesentery, ec = ectoderm of column wall. Camera. Zeiss, D, 2. 



Fig. 22. Transverse section through same mesentery and the adjacent one of the 

 first pair taken 0.08 mm. lower down. Lettering as in preceding figure, e.xcept ms = 

 median streak of mesenterial filament. Camera. Zeiss, D, 2. 



Fig. 23. Transverse section of mesenterial filament of the first pair, from larva 

 from which Fig. 20 was taken. Camera. Zeiss, D, 2. 



Fig. 24. Transverse section of mesenterial filament of mesentery of the first pair, 

 taken just below the bend of the mesentery, from larva with twelve perfect mesen- 

 teries. Lettering as in Fig. 22^ except Is = lateral streaks of mesenterial filament. 



