536 COMPARATIVE ANATOMY CHAP. 



median line, but on the posterior side shifts the more to the right the nearer it 

 approaches the ventral side. This is the principal mesentery. Ventrally the two 

 coelom sacs still remain far apart. 



In the anterior vesicle, the hydroccel rudiment, together with the rudiment of 

 the parietal sinus, becomes separated from the rudiment of the intestine. After 

 this separation has taken place, the hydrocoel vesicle still remains for a short time 

 in open communication with the parietal sinus. 



The hydroccel vesicle lies close below the thickened ventral ectoderm, shifted 

 somewhat far to the left. 



The rudiment of the parietal sinus becomes a transverse tube. 

 The rudiment of the intestine changes shape. It is no longer, as before, an 

 incomplete hollow ring, placed vertically, through which the connecting piece of 



the two coelom vesicles passed (Fig. 445). 

 The connecting piece having degenerated, 

 the lumen of the tubular ring has room 

 to expand from before backward, until 

 the hollow ring becomes a vesicle. 



Towards the close of embryonic 

 development, in the fifth stage, the first 

 rudiment of the calcareous skeleton 

 appears. In an embryo one hundred 

 hours old, the rudiments of the following 

 plates were found : 5 orals, 5 basals, 3-5 



infrabasals, and about 11 segments of 

 FIG. 445. Posterior end of an embryo of ,, , ,, 



Antedon sizty hours old, seen from the right r 



side (after Seeliger). 1, The outline of the right Tlie five orals liave a superficial 



coelom sac ; 2, rudiment of the parietal sinus ; position at the posterior part of the 

 3, rudiment of the hydroccel ; 4, mesenchyme ; embryo, making a horse - shoe - shaped 

 6, ventral, and 7, dorsal process of the , mesentero- arch which ig ant eriorly and down- 



hydroccel vesicle ; 6, connecting duct between the , ' 



right and the left coelom sacs. , wards - The left end of the arch reaches 



further forward than the right. 



As a rule (with the exception of the first oral, which indicates the end of the 

 left side of the arch) the five orals lie round the left coelom sac. 



The five basals have exactly the same arrangement as the five orals, merely lying 

 somewhat further forward than the latter. All of them, except the first basal, lie 

 above the right ccelomic vesicle. 



The 3-5 infrabasals again, which are still extremely small, lie in front of the 

 basals, but further below the surface of the embryo. 



In the anterior half of the embryo the infrabasals are followed by the row of 

 stalk plates. This row forms an arch which is concave towards the ventral surface, 

 so that the most anterior, the pedal, plate, lies near the floor of the vestibular 

 invagination. 



The newly-arising skeletal plates of the stalk appear at the posterior end of the 

 row, generally (but not exclusively) immediately in front of the future centrodorsal, 

 between it and the last formed most posterior stalk plate. 



Up to this time the embryo has lain enclosed in the egg-membrane, on the pin- 

 nule of the mother, but it is now ready to be hatched. Even at this stage its 

 organisation leads to the conjecture that the calyx will be produced from the larger 

 posterior half, which alone contains the internal rudiments, while the stalk of the 

 future attached larva will be produced from the anterior half. 



