OVUM. 



gregated ; the latter being not organically united 

 like the compound polypes, but merely adher- 

 ing more or less strongly to one another so as 

 to form a chain. The aggregated, but not the 

 solitary kind, possess sexual organs; and it 

 would appear, though this is not yet deter- 

 mined with certainty, that all the individuals 

 of one chain are of a similar sex either male 

 or female. 



Fig. 21. 



Solitary and aggregated Salpce. (From Sars.) 



A, solitary Sal pa, with chain of aggregated ones, 

 g, budding from it. 



B, this chain magnified, shewing the successive 

 sets ill different stages. 



c, one of the more advanced aggregated Salpre 

 from a ch;iin, /, the place of a foetus formed by sex- 

 ual generation. 



D, foetus from another more advanced, magni- 

 fied ; h, the yolk, by which it adhered to the pa- 

 rent ; g, the place of the germ for the aggregated 

 chain. 



All the individuals of a chain of aggregated 

 Salpae are produced from a solitary one by a 

 process of internal gemination, or gradual deve- 

 lopment from an internal stolon, or germ-stock, 

 from which they are detached gradually and 

 in successive groups : all the individuals of the 

 chain are contained within a tube, and become 

 united to each other after their development, 

 presenting a series of groups of different de- 

 grees of advancement ; but the individuals in 

 each group being nearly at the same stage of 

 development. 



The distinct or single Salpae, which, with 

 the exception of the want of the sexual 

 organs, do not differ materially from the in- 

 dividuals of the aggregated chain, are produced 

 from fecundated ova which are developed within 

 the body of the parent. These ova differ 

 from the germs from which the aggregated in- 

 dividuals take their origin in the possession of 

 a yolk, and external envelope. Their de- 

 velopment proceeds to its termination within 

 the parent body, and the young Salpa is already 

 provided with the internal stolon for the gem- 

 mation of its chain progeny, before it passes 

 into its separate state ol existence. 



The solitary Salpae may be looked upon, 

 therefore, probably as incomplete or larva 

 forms, and the aggregated are the fully deve- 

 loped sexual individuals. The generation of 

 this animal, therefore, is precisely an example 

 of that succession of two different kinds of 

 individuals which has been distinguished as 

 alternation of generations ; each fecundated 

 ovum of the sexual individuals being developed 

 into an animal which never acquires sexual 

 organs, and which produces by a process ap- 

 parently of the nature of gemmation, a nu- 

 merous brood of individuals associated in a 

 chain ; all of which are sexually perfect, one 

 set developing only spermatozoa, and the 

 females among them being the producers of 

 the ova, which are the source of the new 

 generation.* 



Although no other instances of alternate 

 generation have yet been observed in the 

 class of Mollusca, yet it is possible that modi- 

 fications of this process may hereafter be dis- 

 covered. An observation related by Agassizf, 

 in regard to the development of the ovum in 

 one of the Eolidae, deserves to be recorded, as 

 it may be found to constitute an approach to 

 the metagenetic process. After having de- 

 scribed the usual process of division of the 

 yolk in which the first stages of development 

 consist, and the farther progress of formation 

 in the Eolis, he says, " But the most 

 curious phenomenon which takes place is this ; 

 that the whole yolk does not constantly go 

 to form one single individual. But there may 

 be instances when the mass of yolk, which 

 has been subdivided into cells, is itself di- 

 vided into two or three or more masses, which 

 grow independently, several individual animals 

 arising from one mass of yolk, which thus 

 divides." 



Entozoa. Among the Entozoa the process of 

 reproduction is effected by very various means. 

 All the Nematoidea, or round worms, are 

 of distinct sexes ; and their fecundated ova 

 are developed into the parental form without 

 any metamorphosis of a marked kind, (ex- 

 cepting perhaps in the Echinorrhynchi, the 



* Sec Savigny, Mem. sur les Anim. sans Verteb. 

 1816 ; Chamisso, De Salpis, 1819 ; Meyen, Ueber die 

 Snlpfii; Eschricht, in the Isis, 18-12; Sars, Fauna 

 Littor. Norvegiae, 18-10 ; Krohn, Ann. des Scien. 

 Nat. July, 1846; who first pointed out the existence 

 of spermatozoa in certain individuals of the aggre- 

 gated chain. 



f Lect. on Comparative Embryology, Boston, 

 1849, p. 81. 



