EMBRYOLOGY. 



711 



the first opening formed leading into the stomach {Fig. .:.j). With the for- 

 mation of the depression a slight flattening also occurs. In the next stage 

 (Fig. 34) the thickened Avail bends in forming a shallow depression. This 

 depression goes on, becoming deeper {Fig. .;.;) and deeper {Fig. 36) until it 

 forms a pouch extending half the length of the embryo {Fig. 37) ; the 

 walls of the pouch retaining their thickness, while the walls of the embryo 

 itself become more and more attenuated at each succeeding stage. Water 

 flows freely into and out of this cavity ; currents are established, running in 

 different directions along opposite walls of the pouch, showing this opening 

 for the present to be a mouth, as in the embryo of Asteracanthion, the pouch 

 sustaining the same relation to the whole body as in other Radiates* 



The embryo, on escaping from the egg, resembles a starfish embryo,! and 

 it would greatly puzzle any one to perceive any difference between them. 

 The formation of the stomach, of the oesophagus, of the intestine, and of the 

 water-tubes takes place in exactly the same manner as in the starfish, the 



1'IG. 36. 



Fig. 



time only at which these different organs are differentiated not being the 

 same. We have thus very early in the history of these two orders differences 

 which to a practised eye tell at once to which of them the 3"oung embryo 

 belongs. A particularly important difference is the formation in Ophiurans 

 and in Echinoids of calcareous rods at an early period of the pluteus condition. 

 The small cavity first formed goes on increasing in length until we have a 

 hollow cylinder {d) extending half the length of the embryo, as in Figs. 86 



* So far the changes which have been observed do not differ materially from what wc know of the 

 earlier stages of Echini embryos from Derbes, Miiller, and Krohn. The observations of the earlier 

 stages, as given by Krohn and Derbes, supplement each other, and Miiller has taken up the subject where 

 they left it But this is the only sea-urchin in which we have a complete series in the development as 

 complete as that of the common starfish of our coast, which I published in 1864. 



f See Figs. 4 and 5 of my paper on Asteracanthion, in Proc. Am. Acad., Vol. VI., April 14, 1863, 

 which represent the corresponding stages of the embryo of the starfish, and the figures in PI. I. and II. of 

 the Embryology of the Starfish. 



