Chap. I. THE EMBRYO AFTER HATCHING. 7 



the first axis of segmentation, and are, as far as I have observed them in the seg- 

 mentation of Asteracanthion and of Toxopneustes, always at one pole of the first 

 axis of segmentation. 



The Embryo after hatching. — 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 (PL I. Fig. 21). When the 

 outer envelope is torn, the young rotate slowly about round a shifting axis, by 

 means of very minute cilia placed over the whole surface ; the walls are every- 

 where 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 thickening is accompanied by a flattening of the same side (PL I. Fig. 



23, a) ; the embryo has lost its regular spherical shape and its homogeneous walls 

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

 pole (PL 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 

 e,mbryo (PL I. Figs. 25, 26, d, 27, d). While a cavity (d) is thus formed by the 

 simple folding in of the outer wall, the embryo is constantly lengthening and be- 

 comes more cylindrical; the walls of the extremity opposite the pouch becoming 

 attenuated, while, immediately round the opening of the cavity, the walls have not 

 lost their original thickness (PL 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 whole body as in the 

 most regular and circular radiated animals, such as } r oung Actiniae, or young Porites. 

 The motion of the embryo, which immediately after escaping from the egg is an ex- 

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

 cavity equals half the length of the embryo (PL I. Fig. 27, d), the motion is much 

 accelerated. Instead of a simple slow rotation, with scarcely any motion of transla- 

 tion, 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. 



At the end of about twenty hours after fecundation, the embryo has reached the 

 condition just described; it is now somewhat pear-shaped, with rounded extremities 

 (PL I. Fig. 27), having at one end an opening (a), leading into a pouch (d), which 

 extends half the length of the cylinder. 1 We have now the embryo in a condition 



1 So far, the changes which have been observed vations of Derbes, Mtiller, and Krohn. As I have 

 do not differ materially from what we know of the shown, in the Memoirs of the American Academy 

 earlier stages of Echinoderm larvas, from the obser- for 1864, the earlier stages of the Echinus larvae, 



