THE MORTHOLOGICAL COMPOSITION OF AXIMALS. 105 



of certain doctrines tliat liciye long been dominant, and liave 

 still a wide currency. 



Among the- Vertebrata, as among the MoUusca, liomogenesis 

 is universal. The two sub-kingdoms are like one another 

 and unlike the remaining sub-kingdoms in this, that in 

 all the tj^Des they severally include, a single fertilized ovum 

 produces only a single individual. It is true that as the 

 e2£:s of certain Gasteropods occasionally exhibit spontaneous 

 hssioTi of the vitelline mass, which may or may not result in 

 the formation of two individuals ; so among verteOraie ani- 

 mals we now and then meet with double monsters, which 

 appear to imply such a spontaneous fission imperfectly car- 

 ried out. But these anomalies serve to render conspicu- 

 ous the fact, that in both these sub-kingdoms the normal 

 proces'i is the integration of the whole germ-mass into a 

 single organism, which at no phase of its development dis- 

 plays any tendency to separate into two or more parts. 



Equally as throughout the Moil iisea there holds throughout 

 the Vertebrata, the correlative fact, that not even in its lowest 

 any more than in its highest types, is the body divisible into 

 homologous segments. The vertebrate animal, under its 

 simplest as under its most complex foi^m, is like the mollusc- 

 ous animal in this, that you cannot cut it into transverse 

 slices, each of which contains a digestive organ, a respiratory 

 organ, a reproductive organ, &c. The organs of the least- 

 developed fish as well as those of the most-developed 

 mammal, form but a single physiological whole ; and they 

 show not the remotest trace of having ever been divisible 

 hito two or more physiological wholes. That segmentation 

 ■^hich the vertebrate animal usually exhibits throughout 

 part of its organization, is the same in origin and meaning 

 as the segmentation of a Chiton's shell ; and no more implies 

 in the vertebrate animal a composite structure, than do the 

 successive pairs of branchiae of the Doto or the transverse rows 

 of branchiae in the Eoiis, imply composite structure in the 

 molluscous animal. To some this will seem a very question- 



