28 EMBRYOLOGICAL MONOGRAPHS. 
PLATE IX. 
Development of the Ecutnorpea. Figures from Emm SeLenka, WILHELM Buscu, JOHANNES 
Mutter, J. W. Fewxes, H. Garman and B. P. Couron, and Auexanver AGAssiz. 
1-16. Echinus miliaris. From E. Selenka, Keimblitter und Organanlage der Echiniden, 1880, Pl. V.  Zeits. f. 
Wiss. Zool., XX XIII. 
1. Free swimming blastula, optical section ; a, position of the future anus ; }, funnel-shaped depression ; /, seg- 
mental cavity. 16 hours after artificial fecundation. 
. Blastula with the two clusters of mesoderm cells, m, 7, which have separated from the thickened part of the 
ectoderm ; a, position of the future anus. 18 hours. 
3. Commencement of the invagination. 22 hours. 
. Young gastrula, optical section; «, gastrula mouth (later anus). 27 hours. 
5. Gastrula, optical section ; small limestone rods have made their appearance : w, digestive cavity ; m, m!, acen- 
mulation of cells from which the y-shaped rods take their origin ; c, expansion of the closed extremity of 
the digestive cavity, from which are developed the diverticula forming the water-tubes. 43 hours. 
6. Gastrula, dark mesoderm cells at the upper extremity of the larva ; the diverticulum (water-system) of the 
digestive cavity forms a T across its closed extremity ; the limestone rods have greatly increased in length 
and are surrounded by the nomadic skeleton cells. 48 hours. 
. Gastrula, after 54 hours. Optical section, showing the position of the limestone rods. The digestive cavity, 
p, is becoming differentiated into an cesophagus, stomach, and intestine. The water-system, vp, has com- 
pletely separated from the digestive cavity. Lettering as before. 
8, 9, 10, 11, 12. Successive stages of the digestive cavity and its diverticulum, showing the manner in which the 
water-system is formed as a diverticulum at the blind extremity of the digestive cavity, and how it becomes 
separated from it. This process takes place, according to Selenka, in less than three quarters of an hour. 
13. Pluteus, after 60 hours. It has lost its cylindrical outline, the rudiments of the arms appear, and the oral and 
anal planes of the larva are developing in opposite directions. 
14. Diagram of the same Pluteus as fig. 13, seen in profile ; a, cesophagus ; 8, stomach ; y, intestine. The skele- 
ton is not indicated. 
15. Pluteus 94 hours after fecundation : 0, mouth ; a, anus (gastrula mouth) ; a, esophagus ; 8, stomach ; , in- 
testine; vp, right water-tube ; p, left water-tube, which subsequently becomes the water system of the 
young Echinus and the left peritoneal sac. The cesophagus, a, is capable of a considerable contraction and 
expansion ; the narrow passages and 7, leading from the cesophagus and intestine into the stomach, are 
well seen in the profile figure of the same Pluteus (fig. 16). 
16. The same Pluteus as fig. 15, seen in profile. The cilia which still cover the whole Pluteus are not represented 
in this figure, nor in figs. 13, 14. 
bo 
Cs 
ba | 
17-19. Strongylocentrotus lividus. From BE. Selenka, Keimblitter und Organanlage der Echiniden, 1880, Pl. VIT. 
Zeits. f. Wiss. Zool., XX XIII. 
17. Blastula making its escape from the egg; d, outer membrane. 
18. The same blastula, free, optical section ; en, thickened wall of the blastula at the pole where the invagination 
will take place. 
19. Gastrula 43 hours old. The vasoperitoneal vesicle has separated from the digestive cavity. Optical section, 
the two first y-shaped limestone rods of the skeleton have appeared. 
20-23. Arbacia pustulosa. From FE. Selenka, Keimblitter und Organanlage der Echiniden, 1880, Pl. VII. 
Zeits. f. Wiss. Zool., XX XIII. 
20. Blastula 30 hours after artificial fecundation ; em, thickened wall of the blastula, where the invagination will 
take place ; f, segmental cavity ; g, anal pole. 
