1-A 



OVUM. 



may be detached and live independently, but 

 they nevertheless constitute different animals, 

 and cannot be regarded in any other light 

 than as so many individuals distinct from the 

 one producing them, although all are de- 

 scended from one ovum, or all are necessary 

 to make up the entire species. 



And it is further to be observed that each 

 of these several animals may be subject to in- 

 dividual metamorphosis, and that in some 

 classes there is so gradual a transition from 

 individual change to new production that it 

 may be difficult to determine to which of 

 these forms of reproductive development their 

 phenomena ought to be referred. 



In that part of the article which treats 

 specially of development our attention may 

 again be called to some of the more remark- 

 able examples of individual metamorphosis 

 that are known : at present it is intended 

 rather to bring prominently forward those 

 instances of alternate generation which have 

 been discovered since the publication of the 

 Article GENERATION, or which, if previously 

 known, may now be viewed in a different 

 light, in consequence of being brought into 

 comparison with other observations of a 

 similar kind and of more recent discovery. 



We may first consider some examples of 

 this process, or of one very analogous to it, in 

 which the new animal is single. 



Echinodermata. In several orders of this 

 class a variety of the reproductive process has 

 of late years been pointed out, in regard to 

 which it may be doubted whether it is most 

 of the nature of a metamorphosis or a meta- 

 genesis, but which, as it has been considered 

 by J. Miiller, the discoverer of the most in- 

 teresting and remarkable of its phenomena, as 

 in some measure analogous to the alternating 

 generation, I will mention in this place ; the 

 more so, that it might almost be looked upon 

 as forming the connecting link between the 

 direct and the alternating processes of repro- 

 duction. 



In some of the Echinodermata it appears 

 from the earlier observations of Sars that the 

 young produced from the ova are developed 

 directly into the parental form, passing how- 

 ever through several marked modifications in 

 the early stages of development. Thus, some 

 of the star-fishes (Asteracanthion gtarialis, 

 Sars) leave the egg as a ciliated free moving 

 animalcule, then they become pediculated and 

 attach themselves, have four club-shaped pro- 

 cesses developed on them, and, lastly, they 

 pass by the development of the rays and the 

 internal organs into the complete form ; but 

 here the whole, or nearly the whole, germinal 

 mass of the ovum is converted into the embryo 

 or larva, and the whole, or nearly the whole, 

 of this undergoes the farther changes of con- 

 version into the complete and sexual animal.* 



From the researches of J. Miiller it ap- 



* Sars, Fauna Littor. Norvegiae, 1846 ; and Ann. 

 des Sciences Nat. ; Agassiz, Lectures on Comparative 

 Embryology, New York, 1849 ; and a Letter from 

 Desor to J. Mttller, in Archiv. fur Physiol. 1849, 

 p. 79. 



pears that the mode of development now 

 described is exceptional among the Echino- 

 dermata, and that in other families of the erder 

 Asteriadas, and in the Ophiura and Echinidoc, 

 an embryo or larva of a peculiar kind, is 

 formed by direct development from the fe- 

 candated ovum, which is not itself converted 

 into the complete animal, but rather serves as 

 a temporary stock from which the perfect 

 animal is subsequently formed in a manner 

 that may be compared to gemmation. But it 

 does not appear that more than one individual 

 is developed from each primary larva stock, 

 and this gradually dies away, so soon as its 

 attached offspring has made some advance in 

 its formation. This body, described under 

 the name of Bipinnaria asterigera, as con- 

 nected with an Asterias, is a comparatively 

 large animal, with a long pediculated body, 

 twelve or fourteen tentacles, an alimentary 

 canal, consisting of mouth, gullet, stomach, in 



Fig. 7. 



Bipinnaria. asterigera (from Midler}. 



A, the young larva before the Echinoderm is 

 formed. 



B, a more advanced larva, with the Asterias on 

 its summit. 



c, the Asterias torn up to show its stomach, a 

 continuation of the alimentary canal of the larva. 



