no COMPARATIVE ANATOMY 



become common to speak of a physiological division of labour 

 amongst the cell-castes, with a corresponding morphological 

 differentiation of form. The illustration is a useful one, but 

 it ceases to be valuable if pushed too far. The cells compos- 

 ing the frog's body are not independent units in the sense that 

 the individual men composing a colony are individual units. 

 Their life not only contributes to the life of the whole, but 

 is indissolubly connected with it; and in this connection it 

 should be borne in mind that, however much the other func- 

 tions of an individual cell may be depressed in order that 

 its special function may be exalted, all cells retain and must 

 retain one vital attribute, that of assimilation. All cells, whilst 

 they retain their vitality, must be nourished, and they are 

 nourished by the blood through the intermediary of the 

 lymph. Each cell has its own kind of metabolism ; but the 

 metabolism of each, whilst it contributes to the whole, is 

 merged into the metabolism of the whole, and ceases when 

 it is removed from it, except in one case, to be considered 

 directly. 



All these considerations and facts are collected together in 

 what is known as the Cell-Theory, which was first propounded, 

 for the vegetable kingdom, by Schleiden, a botanist, in 1838. 

 Schleiden's views were adopted and applied to the animal 

 kingdom by Theodore Schwann in 1838 and 1839. 



The continuous researches of botanists and zoologists have 

 added much to the cell-theory since the days of Schwann, and 

 at the same time have rendered his position less tenable. 

 Schwann made a fundamental error regarding the genesis of 

 cells. He supposed that they were formed, in a manner 

 analogous to crystallisation, out of a nutrient matrix, which 

 he called the cytoblastema. He supposed that the cell was a 

 hollow vesicle containing another vesicle, the nucleus, and that 

 this again surrounded a smaller vesicle, the nucleolus. The 

 nucleolus, according to his views, was first deposited in the 

 structureless or minutely granulous cytoblastema, and it formed 

 a centre of attraction round which other molecules were 

 deposited to form the nucleus. This, again, continued to attract 

 fresh molecules, which formed the cell-body. 



The error of this view was pointed out in 1851 by Robert 

 Remak, who showed that all the cells of which an animal body 

 is composed are formed by the continuous and repeated sub- 



