50 ALTERNATE GENERATION 



of generation, wliicli are not distinct till the end of the 

 second week. 



Excepting slight differences in the length of the in- 

 tervals in which the animal arrives at the various stages 

 of its development, the same mode of development takes 

 place in all the compound Ascidia, and in all of them 

 consequently, the progeny consists of solitary individuals, 

 which grow up into solitary Ascidice. 



With regard to the mode in which the colonies are 

 formed, especially when they are composed of individuals 

 distributed in regular groups as in the genus Botryllus, 

 &G., the question, as it appears to me, has been far too 

 lightly treated by M. Edwards. Eor he has observed, that 

 among the Ascidi<s composites, and sociales, individuals 

 occur which propagate themselves by gemmation as it is 

 commonly termed, since from the lower end of the animal 

 peculiar processes arise, which gradually become irre- 

 gularly or dichotomously branched, whilst the enlarged 

 extremities of these branches are, at a later period, formed 

 into Ascidice. Since in this way from a single individual 

 several Ascidice may originate, M. Edwards supposes that 

 the colonies owe their origin to solitary individuals, which 

 have multiplied themselves by gemmation, and from 

 which new individuals successive gemmations again take 

 place until the whole number is formed, which we find 

 constituting a given mass or group. To this it may be 

 fairly objected, that although in the ascidian groups or co- 

 lonies, the separate individuals are able to multiply them- 

 selves by gemmation, and although it may be admitted as 

 in some degree analogous to this, that it is possible that 

 the solitary Ascidice, into which the caudate young pro- 

 duced from the egg of the aggregated Ascidice are changed, 

 may throw out similar gemmce — (although, so far as I 

 know, this has not been observed) — still, individuals of 

 equal size and similar form, do not spring at one and the 

 same time from such gemmation, and still less, do groups 

 of individuals, arranged in a definite number around a 

 common centre or central line, h^iw Botryllus, &c.; which 



