COMPARED WITH THOSE OF ANIMALS. 35 



of plants, present the same uniformity of appearance in 

 their external configuration. Some of them maintain this 

 condition throughout the life of the animal, and are the in- 

 struments by which the strictly vital operations are carried 

 on ; others rapidly undergo a change of form in accordance 

 with those laws of growth to which they are individually 

 subject. In this respect precisely, the same laws govern 

 both the animal and vegetable world. " A globular mass," 

 says Carpenter, " containing a large number of cells is 

 formed before any diversity of parts shows itself ; and it is 

 by the subsequent development from this mass of different 

 sets of cells, of which some are changed into cartilage, others 

 into nerve, others into muscle, others into vessels, and 

 so on, that the several parts of the body are ultimately 

 formed." 



There is, however, one distinction between the cells of 

 plants and animals which must not be overlooked. It con- 

 sists in the fact that the cells of plants are rnu^h larger than 

 those of animals, and retain all the characteristics of cells 

 throughout the life of the plant, so that a cross section of 

 any part of the vegetable fabric will at any time show them. 

 But the cells of animals rapidly undergo a development into 

 tissues in which the cellular form wholly disappears. Hence 

 it is that the cellular origin of many of the .animal tissues 

 can only be detected in the ovum ; in the fully developed 

 embryo all appearance of cell and nucleus has vanished. 

 Thus whilst the cellular origin and structure of plants has 

 been long known, that of animals is to be enumerated 

 amongst the discoveries of modern times. 



As an instance of this gradual obliteration of the nucleus 

 and cell wall, we refer to those cells which originate the 

 more permanent and solid parts of the animal body ; such, 

 for example, as the teeth of man, or the shells of the mol- 



