257 



in its posterior |);ul llu' \v;ill already lias assuiiu'ii an endothelial character. The 

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

 Fig. 2 1. From a series of loiigiludinal, frontal sections of an embryo, .'$2 — 3.") hours old; 

 rif>. 2 is the more dorsal: fig. I the more ventral of them. The pharynx has been 

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

 The lu-hinold-rudinieiit has begun to form lobes, representing the young spines 

 and prinuiry tubefeet. In fig. I 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 ap|)ear like a canal. This is, however, 

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

 - ^^. Median longitudinal sagittal section of a larva, 21 22 hours old. The amniotic 

 prolongation of the pharynx has reached the i)oslcrior end, covering the whole of 

 the dorsal side (comp. I'l. XV. Figs. 20 — 21). Its dorsal wall is beginning to assume 

 an endothrlial character, while the ventral wall has already thickened considerably. 

 1)1. blastocoel : h. hydrocoel: i. irdesline. 



6 — 7. From a series of longitudinal, sagittal sections of a larva, 2(i — 29 hours old. The 

 anuiiotic opening, the fornu-r mouth, has now been Iransplaced 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 lluin being the coelom (c). 



8 — 9. Fi-(,i,i a series of longitudinal, sagittal sections of a larva, 32—3.") hours old; fig. 8 

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

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

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

 limited part of the coelom in fig. 9 is also seen in the adjoining section lo be in direit 

 conmiunicalion with the rest of the coelom. 



- 10. Median longituifinal, sagittal section of a larva, 11 — 13 hours old. The anuiiotic 



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

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

 bl. blastocoel; cnt. entoderm: ph. pharynx: r. rectum. 



- 14 — 16. From a series of transverse sections of a larva, 23-21 hours old. Fig. 11 the 



more anlerior, 16 the more posterior. In fig. lo is seen the last trace of the blasto- 

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

 and 16 by 1 sections. The beginning folding of the ventral wall of the amnion is 

 to be noticed, am. amnion; bl. blastocoel. The cavity in figs. 11 and !.'>, design- 

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



- 17 — 18. F'rom a series of transverse sections of a larva, 32 — 3.^ hours old. The young 



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

 separated by 8 sections, fig. 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 figures are separated by 4 sections, fig. 19 being the 

 more anterior. 



Plate XVII. 

 .\11 figures of Heliocidaris eriillirof/ramma (Val.). .Ml '^/i. 

 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 difTerentiation of the arclunleron. in the 

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



4. l-'urther advanced stage in the dilTerentiation of the arclienlerou, a large pouch 

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



'). Embryo, 42 hours old, showing the first indication of the amniotic invagination. 

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

 by a thill strand, the space seen above the arclienlerou is the tiydro-enterocoel. 



33 



