118 MORPHOLOGICAL DEVELOPMENT. 



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

 formation of two individuals; so among vertebrate animals 

 we now and then meet with double monsters, which appear 

 to imply such a spontaneous fission imperfectly carried out. 

 But these anomalies serve to render conspicuous the fact, 

 that in both these sub-kingdoms the normal process is the 

 integration of the whole germ-mass into a single organism, 

 which at no phase of its development displays any tendency 

 to separate into two or more parts. 



Equally as throughout the Mollusca, there holds through- 

 out the Vertebrata the correlative fact, that not even in its low- 

 est 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 form, 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 

 into two or more physiological wholes. That segmentation 

 which the vertebrate animal usually exhibits throughout 

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

 as the segmentation of a Chitons 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 Eolis, imply composite structure in 

 the molluscous animal. To some this will seem a very ques- 

 tionable proposition; and had we no evidence beyond that 

 which adult vertebrate animals of developed types supply, it 

 would be a proposition not easy to substantiate. But abundant 

 support for it is to be found in the structure of the vertebrate 

 embryo, and in the comparative morphology of the Vertebrata 

 in general. 



Embryologists teach us that the primordial relations of 

 parts are most clearly displayed in the early stages of evo- 



