MEM. M. C. Z., 1X. No. 2,—ECHINODERMATA. 31 
o ca Dp: WD. Ge 
Development of the ECHINOIDEA, continued. Figures from ALEXANDER AGassiz. 
a. Anus. s!. Pigment spots of the long Pluteus arms. 
b. Madreporie body. s'.  Interambulacral spines of young Echinus. 
c. Alimentary canal (intestine). s!!, Young spines of embryo Echinus. 
d. Digestive cavity (stomach). t. Tentacles. 
e/, ell, ell, xv. Arms of the Pluteus. Up Odd terminal tentacle. 
f.  Brachiolar arms ? di, 2. First and second pairs of lateral ambulacral 
k. Teeth of young Echinus. tentacles. 
m. Mouth. v Anal part of vibratile cord. 
ms. Mesoderm cells. v!. Oral part of vibratile cord. 
mn.  Interambulacral tubercles of young Echinus. v!!,  Vibratile epaulettes. 
n!. Ambuiacral tubercles of young Echinus. w. Water-tube. 
o. (Esophagus. w!. Water-tube communicating with the madre- 
p.  Pedicellarie. poric body. 
v!. First y rod of the Pluteus. 
1-17. Strongylocentrotus Drébachiensis. From Alexander Agassiz, Revision of the Echini, Pt. 1V., 1874. Ilust. 
Cat. Mus. Comp. Zool., No. VII., pp. 709, 710. 
18-44. Strongylocentrotus Dribachiensis, continued. From Alexander Agassiz, On the Embryology of Echino- 
derms. Mem. Am. Acad., IX., 1864, selection from figs. 1-25. 
1, Mature egg. 
2. Egg in which the germinative vesicle has disappeared after fecundation. 
3. The germinative dot has also disappeared. The yolk has separated from the outer membrane, leaving an 
interval between it and the envelope. 
4, The yolk has become depressed at one pole. 
5. The same depression takes place at the two poles. 
6-12. Different stages of segmentation. 
6. The first trace of segmentation is a slit at one pole. 
7. The yolk is divided into two large ellipsoidal masses. 
8. The masses subdivided again. 
9. The four spheres show a tendency to a further subdivision. 
10. There are now eight spheres of segmentation. 
11. There are thirty-two spheres, and they already show a tendency to form an envelope. 
12. There are sixty-four spheres, and the walls of the embryo are already indicated. 
13. The segmentation has gone on until the spheres are quite small, and the wall of the embryo very distinct. 
14. The embryo has just escaped from the egg ; the wall is thickened at one pole. 
15. A slight invagination forms at the thickened pole. 
16. The invagination has become somewhat deeper. 
17. Older embryo more elongated, with thin walls at the upper extremity; the invagination now performs the 
function of a rudimentary digestive cavity. 
18. Still older embryo, seen in profile ; the digestive cavity is no longer in the axis of the Pluteus. 
19. The same as fig. 18, seen from above. 
20. Profile view of a Pluteus, somewhat more advanced than fig. 18 ; the digestive cavity is bent towards the ven- 
tral side of the larva. 
Somewhat older Pluteus, at the end of the fourth day, seen from above. At the closed extremity of the 
digestive cavity two small diverticula have formed (w, w!), the first appearance of the water-tubes. 
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