2 ON THE EMBRYOLOGY OF ECHINODERMS. 



observed in larvae obtained by artificial fecundation. He succeeded in tracing them 

 for about three weeks, which is not quite as long as I have kept them alive. Miiller 

 has unfortunately not given us any figures of the very first stages of this species, nor 

 has he found the adult larvae swimming about immediately before the absorption of the 

 Pluteus. The series of figures found in this paper will give us a more complete idea 

 of the difierent phases of growth of one species of Echinus, than can be gathered from a 

 comparison even of the different species which Miiller has investigated. It will enable 

 us to trace the order of appearance of the arms of the Pluteus, and the last changes 

 which the larva undergoes immediately before the young Sea-urchin has resorbed the 

 whole framework of the Pluteus. 



The process of segmentation of the yolk is entirely similar to what we observe in 

 the Starfish ; the main differences in the eggs are simply of proportion between the 

 relative size of the yolk-mass and the outer envelope. My observations agree with 

 the account of the segmentation given by Derbes.* The formation of the " Richtung's 

 Blaschen " is very easily followed in the Sea-urchin. 



The yolk contracts somewhat immediately after the fecundation takes place, and we 

 might repeat here, word for word, the description of the changes which the yolk under- 

 goes in the Starfishes, and have the history of the changes during the segmentation of 

 the yolk of an Echinus. For an account of this I would refer the reader to the fifth 

 volume of the Contributions to the Natural History of the United States, by Professor 

 Agassiz. The embryo, on escaping from the egg, resembles a Starfish embryo, and it 

 would greatly puzzle any one to perceive any diff"ereuce between them. The forma- 

 tion of the stomach, of the oesophagus, the intestine, and the water-tubes takes place 

 in exactly the same manner as in the Starfish, the time only at which these diff'erent 

 organs are diff'erentiated 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 young larva belongs. What is a particularly important difference is the forma- 

 tion in Ophiurans and in Echinoids of calcareous rods at an early period of the larval 

 condition. 



The emhryo after its escape from the egg. 



In the spherical embryo soon after its escape from the egg we perceive a thicken- 

 ing of the walls at one of the poles ; a depression is then formed at this extremity, 

 which becomes more and more marked ; ■\ the wall then turns in, and a small cavity 



• DERBi;s. Ann. d. Scien. Nat. 3" Ser. VIII. p. 80. 1847. 



t See figs. 4 and 5 of my paper on Asteracantbion in Proc. Am. Acad., Vol. VI., April 14, 1863, which 

 represent the corresponding stages of the embryo of the Starfish. 



