THE EMBRYO AFTER HATCHING. 9 



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 (PI. I. Flfj. 27), having at one end an opening 

 (a), leading into a pouch [d), which extends half the length of the cylin- 

 der.* We have now the embryo in a condition which can best be com- 

 pared to the embryos of other Radiates; for there is as yet nothing of 

 the complication hereafter introduced in the subject by the development 

 of bilateral parts, obscuring the plan upon which the embryo is built. 

 It is an embryo closely resembling those of the other Radiates, in which, 

 however, the class-characters, distinguishing it from the embryos of the 

 other classes of the type, are already developed beyond question. In 

 the young Polyps the earliest appearance of the class-characters is de- 

 noted by the presence of a few radiating partitions, dividing the cavity 

 of the embryo into distinct chambers. In the Acalephs, in the most 

 rudimentary stages, we already find the chymiferous tubes pushing their 

 way through the spherosome ; while in our larvse the echinodermoid 

 class-character, that of having distinct walls, forming the different organs, 

 is already plainly visible from the mode of formation of this digestive 

 cavity. What unites all these embryos in one great type is, that we 

 have in them all an axis around which are arranged the different elements 



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

 the earlier stages of Echinoderm larvae, from the observations of Derbes, Miiller, and Krohn. As I 

 have shown, in the Memoirs of the American Academy for 1864, the earlier stages of the Echinus larvae, 

 as they have been figured by Derbes, agree in the main points with what has been observed of the 

 earlier stages of our American Echinus larvse (Toxopneustes drobachiensis). With the exception, 

 however, that Derbes, not having followed all the intermediate stages between his figures 15 and 16 

 in the Annales des Sciences Naturelles for 1847, did not see the transformations the digestive cavity 

 undergoes, and committed, therefore, the very natural mistake of supposing that the first-formed opening, 

 which we have described as a mouth, retained the same function afterwards. He, however, correctly 

 noticed the separation of the three cavities, the oesophagus, the stomach, and the alimentary canal, 

 into which this primary cavity is gradually differentiated, and has given a correct iL-scription of their 

 relation to each other. Miiller has taken up this same subject rather where Krohn and Derbes have left 

 it, and although he has traced the development from the egg of several Echinoderm larva, yet he haa 

 not given us as detailed descriptions and figures of the earlier stages, as of those which were more 

 advanced, and says simply, that in the main points his observations coincided with those of Krohn 

 and Derbes. Krohn, Avho has artificially fecundated Echinus lividus, gives us in his figures some of 

 the missing links in the chain of the observations of Derbfes, and shows distinctly for E. liMdus, 

 that the first-formed opening becomes the anus eventually, and in what way this is brought about by 

 the bending of the bottom of the digestive cavity towards one side of the larva, as is the case in our 

 Starfish, and the formation at that point of a second opening, whiclx becomes the true mouth, while 

 the first-formed opening henceforth assumes the function of an anus. 



