257 



in its posterior purl I he \vnll :ilre;nl> lias :issiiinetl an cndothelial character. The 

 spaces below the amis are I he remnants of the blastococl cavity (I)!.). 



Fig. 2 1. From a series of longitudinal, frontal sections of an embryo. :!'_! :,'< hours old; 

 tin- 2 is the more dorsal; fig. 1 the more ventral of them. The pharynx has hern 

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

 The Kchinoid-rudimcnt lias bc^un lo form lobes, representing the young spines 

 and primary tubcfeet. In fig. 1 the curved intestine (i) is seen. The distinctly 

 limited space lo the right of it is part of the eoelom (cl. as is also the large, irregular 

 space in lig. 3. In this latter figure is seen lo 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 (here is no real canal. 

 ."). .Median longitudinal sagittal section of a larva, 21 22 hours old. The amniotie 

 prolongation of the pharynx has readied the posterior end. covering the whole of 

 I he dorsal side (comp. II. XV. l-'i^s. 2u -21). Its dorsal wall is beginning to assume 

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

 bl. blaslocoel; h. hydrocoel: i. intestine. 



67. From a series of longitudinal, sagittal sections of a larva, 2(1 -- 2'.i hours old. The 

 amniolic opening, the former mouth, has now been transplaced lo the dorsal side. 

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

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

 the space separating them being the eoelom (c.). 



<S 9. From a series of longitudinal, sagittal sections of a larva, 32 3.~> hours old: lig. S 

 median, fig. more lateral, separated from fig. cS by Id sections. The two parts of 

 the eoelom seen in fig. S are found to 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 eoelom in lig. i) is also seen in the adjoining section to be in direct 

 communication with the rest of the eoelom. 



10. Median longitudinal, sagittal section of a larva, 41 13 hours old. The amniotie 

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



11 13. From a scries 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 (he 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 scries of transverse sections of a larva, 23-21 hours old. Fig. 14 the 

 more anterior, 16 the more posterior. In fig. !."> is seen the last trace of the blasto- 

 porus, or anal opening (a.). Figs. 1 I and lf> are separated by 1 section, figs. !."> 

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

 to be noticed, am. amnion: bl. blastocoel. The cavity in ligs. 11 and 1.~>, design- 

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



17 18. From a series of transverse sections of a larva, 32 -3.~> hours old. The young 

 spines and primary lubefeel (t.) protruding into Hie amnion. The two figures are 

 separated by S sections, fig. 17 being the anterior, am. amnion; c. eoelom; h. hydro- 

 coel; i. intestine. 



19 20. From a series of transverse sections of a larva, II 13 hours old. Showing the 

 pore canal (p. c.). The two figures are separated by I sections, fig. Ill being the 

 more anterior. 



Plate XVII. 



All figures of Hrlioddarix erythrogranvna (Val.l. All "/,. 

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



2. Kmbryo. 30 hours old, in the gastrula slai^e. 



3. Kmbryo, 30 hours old. showing beginning differentiation of the archenleron. in the 

 lumen of which is seen a mass of a slimyc.'i substance. 



1. Further advanced stage in the differentiation of the archenleron, a large pouch 

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



.">. Kmbryo. 12 hours old, showing the first indication of the amniotie imagination. 

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

 b\ a thin strand. The space seen above Ihe archenteron is the hydro-enterocoel. 



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