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EGHINODEBMATA— ONTOGENY 



535 



The two horns of the anterior vesicle grow out towards one another round the 

 posterior vesicle until they touch, and so form a hollow ring surrounding the 

 posterior vesicle, but not closed posteriorly. 



The posterior vesicle (enteroccel vesicle) assumes the shape of a dumb-bell, its 

 two lateral parts swelling up, while the transverse connecting piece becomes 

 narrower. It is this connecting piece which 

 becomes encircled by the anterior vesicle 

 (Fig. 444). 



The ectoderm thickens on the ventral side. 



The germ, which till now is approximately 

 spherical, begins to lengthen from before hack- 

 ward (in the direction of the principal axis). 



The anterior vesicle forms a large ventrally 

 directed outgrowth, the first rudiment of the 

 hydrocoel (Fig. 445, 3). A small outgrowth 

 of its anterior wall is the rudiment of a sinus, 

 which has been called by some the parietal 

 cavity, and by others the anterior enteroccel (2). 

 The circular anterior vesicle itself becomes the 

 intestine (5, 7). 



In the posterior (enteroccel) vesicle the two 

 lateral swellings increase in size, while ithe 

 connecting piece becomes thinner and thinner, 

 and finally, at a later stage, entirely disappears. 

 The enteroccel vesicle is thus divided into a 

 right and a left enteroccel sac. 



During the next period, which more or less 

 corresponds with the fourth day of develop- 

 ment, the embryo increases somewhat more in 

 length. Anteriorly, in the frontal region, i.e. 



Fig. 444. — Horizontal longitudinal 

 section through an embryo of Antedon, 

 fifty - seven hours old (after Seeliger). 

 1, Point at which the neural plate becomes 

 differentiated ; 2, ectoderm ; 3, mesen- 

 chyme ; 4, place of formation of the 

 mesenchyme ; 5, rudiment of the intes- 

 tine ; 6, rudiment of the left ccelom ; 

 7, ventral outgrowth of the raesentero- 

 . . hydroccel vesicle ; 8, rudiment of the right 



at the end of the embryo diametrically opposite eoelom ; 9, transverse duct, connecting the 

 to the point where the now vanished blastopore two rudiments of the cwlom. 

 lay, a ciliated tuft forms. Ciliated rings 



appear in the arrangement characteristic of the free-swimming larva {cf. Fig. 402, 

 p. 510). 



The ectoderm which carries the neural tuft thickens (neural plate), becomes 

 multilaminar, and at the same time appears to be slightly depressed (Figs. 446 and 

 447). The deep cells become ganglion cells, and nerve fibrillse also appear below 

 the surface, closely applied to the neural plate and formed by the ectoderm ; these 

 are the rudiments of the larval nervous system. 



Ventrally from the neural plate, close behind it in the median line, a pit-like 

 depression forms ; this is the adhesive pit, so called because, at a later stage, the 

 free-swimming larva attaches itself by means of it. 



Another depression, which rapidly deepens and increases in circumference, lies 

 in the thickened ventral ectoderm, and is the rudiment of the vestibule, whose signi- 

 ficance will be explained later. 



The two coelom sacs have become completely detached, the connecting piece 

 having disappeared. That on the right spreads chiefly dorsally forward into the 

 segmentation cavity and then over the intestine, even crossing the median line on to 

 the left side. The left ccelom sac, however, spreads chiefly backward and grows 

 round the intestine posteriorly like a cap, until it touches the posterior wall of the 

 right sac. Dorsally it touches the latter somewhat to the left of the median line, 

 and a mesentery is thus formed which runs dorsally somewhat to the left of the 



