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in its posterior part the wall already has assumed an endothelial character. The 

 spaces below the arms are the remnants of the blastocoel cavity (bl.). 

 Fig. 2 — 4. From a series of longitudinal, frontal sections of an embryo, 32 — 35 hours old; 

 flg. 2 is the more dorsal; fig. 4 the more ventral of them. The pharynx has been 

 very much shortened and now has simply the character of the opening of the amnion. 

 The Echinoid-rudiment has begun to form lobes, representing the young spines 

 and primary tubefeet. In flg. 4 the curved intestine (i) is seen. The distinctly 

 limited space to the right of it is part of the coelom (c), as is also the large, irregular 

 space in fig. 3. In this latter figure is seen to the left below the anterior part of the 

 amnion some nuclei, serially arranged so as to appear like a canal. This is, however, 

 a casual arrangement of no importance and there is no real canal. 



5. Median longitudinal sagittal section of a larva, 21 — 22 hours old. The amniotic 

 prolongation of the pharynx has reached the posterior end, covering the whole of 

 the dorsal side (comp. PI. XV, Figs. 20 — 21). Its dorsal wall is beginning to assume 

 an endothelial character, while the ventral wall has already thickened considerably, 

 bl. blastocoel; h. hydrocoel; i. intestine. 

 6 — 7. From a series of longitudinal, sagittal sections of a larva, 26 — 29 hours old. The 

 amniotic opening, the former mouth, has now been transplaced to the dorsal side. 

 There are two sections between the two figures. Below the thickened ventral wall 

 of the amnion is seen the hydrocoel (h.), and on the ventral side the intestine (i.), 

 the space separating them being the coelom (c). 

 8 — 9. From a series of longitudinal, sagittal sections of a larva, 32 — 35 hours old; flg. 8 

 median, flg. 9 more lateral, separated from flg. 8 by 10 sections. The two parts of 

 the coelom seen in fig. 8 are found to be in direct connection in more lateral sections. 

 The space in the middle probably belongs to the hydrocoel (h. ?). The distinctly 

 hmited part of the coelom in flg. 9 is also seen in the adjoining section to be in direct 

 communication with the rest of the coelom. 



10. Median longitudinal, sagittal section of a larva, 41 — 43 hours old. The amniotic 

 opening is now entirely dorsal. The primary tubefeet are protruding into the amnion. 



11 — 13. From a series of transverse sections of a larva 18 — 20 hours old; fig. 11 is from 

 the anterior end, the two lateral wings being the base of the postoral arms; figs. 12 

 and 13 from the middle part of the body, separated by three sections, am. amnion; 

 bl. blastocoel; ent. entoderm; ph. pharynx; r. rectum. 



14 — 16. From a series of transverse sections of a larva, 23 — 24 hours old. Fig. 14 the 

 more anterior, 16 the more posterior. In flg. 15 is seen the last trace of the blasto- 

 porus, or anal opening (a.). Figs. 14 and 15 are separated by 1 section, figs. 15 

 and 16 by 4 sections. The beginning folding of the ventral wall of the amnion is 

 to be noticed, am. amnion; bl. blastocoel. The cavity in figs. 14 and 15, design- 

 ated h. c, apparently represents both hydrocoel and coelom, not yet separated. 



17 — 18. From a series of transverse sections of a larva, 32 — 35 hours old. The young 

 spines and primary tubefeet (t.) protruding into the amnion. The two figures are 

 separated by 8 sections, flg. 17 being the anterior, am. amnion; c. coelom; h. hydro- 

 coel; i. intestine. 



19 — 20. From a series of transverse sections of a larva, 41 — 43 hours old. Showing the 

 pore canal (p. c). The two flgures are separated by 4 sections, flg. 19 being the 

 more anterior. 



Plate XVII. 

 All flgures of Heliocidaris erythrogramma (Val.). All "/j. 

 Fig. 1. Embryo, 18 hours old, showing beginning formation of the gastrula. 



2. Embryo, 30 hours old, in the gastrula stage. 



3. Embryo, 30 hours old, showing beginning differentiation of the archenteron, in the 

 lumen of which is seen a mass of a sUmy(?) substance. 



4. Further advanced stage in the differentiation of the archenteron, a large pouch 

 having been formed: the hydrocoel (and enterocoel?); 30 hours old. 



5. Embryo, 42 hours old, showing the flrst indication of the amniotic invagination. 

 The archenteron (the dark body in the middle) is connected with the blastoporus 

 by a thin strand. The space seen above the archenteron is the hydro-enterocoel. 



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