8 EMBRYOLOGY OF THE STARFISH. 



Asteracanthion and of Toxopneustes, invariably at one pole of the first 

 axis of segmentation. 



The Emhryo after IlaicUng. — At about the end of the tenth hour after 

 fecundation, the segmentation has been carried so far that the walls of the 

 future embryo have become quite conspicuous, and it is now ready to hatch 

 (PI. I. Fig. 21). When the outer envelope is torn, the young rotate slowly 

 about, around a shifting axis, by means of very minute cilia placed over 

 the whole surface; the walls are everywhere of the same thickness, and 

 the embryo is perfectly spherical. A difference soon becomes evident ; the 

 walls thicken at one pole of the sphere (PI. I. Fig. 22, a), and the thick- 

 ening is accompanied by a flattening of the same side (PI. I. Fig. 23, «); 

 the embryo has lost its regular spherical shape and its homogeneous walls 

 (PI. I. Fig. 23, a). The next change consists in a slight depression at this 

 flattened pole (PI. I. Fig. 24, a); the wall bends inward, forming a very 

 shallow depression, growing deeper and deeper, until it forms a pouch 

 extending half the length of the embryo (PI. I. Figs. 25, 26, d, 27, d). 

 [This stage has become well known as the gastraja stage of Haeckel ; for 

 a fuller discussion of the gastrtea theory see my Memoir on the Embry- 

 ology of the Ctenophorje, Mem. Amer. Acad., 1874, p. 379.] While a 

 cavity (rf) is thus formed by the simple folding in of the outer wall, the 

 embryo is constantly lengthening and becomes more cylindrical ; the walls 

 of the extremity opposite the pouch becoming attenuated, while, imme- 

 diately round the opening of the cavity, the walls have not lost their 

 original thickness (PI. I. Figs. 26, 27, a). 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 to be for the 

 present a mouth ; the pouch, or digestive cavity, sustains the same relation 

 to the Avhole body as in the most regular and circular radiated animals, 

 such as young Actinise, or young Porites. The motion of the embryo, 

 which immediately after escaping from the Qg'g is an extremely slow 

 rotation, increases in rapidity as it lengthens, and by the time the cavity 

 equals half the length of the embryo (PI. I. Fig. 27, d\ the motion is 

 much accelerated. Instead of a simple slow rotation, with scarcely any 

 motion of translation, the latter is now quite rapid, and is accompanied 

 by a slow rotation round a vertical axis, through the centre of the longer 

 diameter of the animal ; the opening leading into the coecum is foremost 

 during; their motion. 



