5G EMBRYOLOGY OF THE STARFISH. 



morphoses of Insects. There is a sort of general similarity between this 

 process of resorption and the growth and changes in the chrysalis of 

 Lepidoptera, ending in a butterfly. In the latter case, the chrysalis, 

 though retaining its character throughout the whole growth and develop- 

 ment of the Insect, has an earlier stage when it seems to be purely chrysa- 

 lis, and a later one immediately before the hatching of the perfect Insect, 

 when the butterfly seems to be gaining the ascendency, and the whole 

 outline of its form may be seen through the chrysalis, which now seems 

 to be only its envelope. And yet the character of the development of the 

 Starfish during its Brachiolaria stage recalls also vividly the phenomena 

 of alternate generations. It is, nevertheless, strictly echinodernioid, and 

 whether we observe it in the Ophiurans, the Sea-urchins, or the Holo- 

 thurians and Crinoids, there seems no doubt, from the observations of 

 Miiller, Busch, Thomson, Krohn, and Agassiz, that it is carried on according 

 to one and the same plan in all the orders of the class, where we have 

 corresponding differences in their various modes of development. With 

 reference to the separate existence of the larva and of the Echinoderm, 

 urged by other observers, I can only say that nothing of the kind has oc- 

 curred in those Echinodenris the changes of which I have ti'aced, whether 

 it be an Ophiuran, an Echinus, a true Starfish, or a Holothurian. 



RECAPITULATION. 



I shall, in a few words, recapitulate the development of these Star- 

 fishes, in order to be able more fully to compare my observations with 

 those of previous writers, and to explain the differences, when they exist. 



Changes of the Yolk. — The yolk, after fecundation, separates slightly 

 from the outer envelope. The segmentation takes place rapidly; as soon 

 as the yolk has divided into eight portions, they arrange themselves in 

 such a manner as to enclose the remaining space, which is more and 

 more separated as the spheres increase in number, until, finally, there is 

 a complete envelope formed of spheres of segmentation. 



Sct/phistoma, or pyriform Stage. — At the time the young escapes from 

 the egg, it is spherical, and the walls of the envelope are of the same 

 thickness. 0«e side becomes thicker, the embryo flattening at this ex- 

 tremity, which is bent in so as to form a slight cavity, in which fluids 

 circulate. This cavity extends half-way the length of the larva, then 



