KINDS OF CELLS. 215 



The animal textures, like the vegetable, originate to a great 

 extent from cells. Such cells, remaining as distinct corpuscles 

 in the fluids, and grouped together in the solids, continuing in 

 some with little change, in others undergoing a more or less 

 complete transformation, give rise to the varieties of form and 

 structure met with in the animal tissues. Even the germ 

 from which an animal originally proceeds is a distinct cell, 

 and the embryo in its earliest stages is but a cluster of cells 

 originating from that primordial cell ; while no distinction of 

 texture is discovered, till a process of transformation of the 

 cells has entered on its progress. In short, reproduction is in 

 the strictest analogy with nutrition, so as to be really but a 

 special form of the great office of assimilation. 



Cells are divided into proper cells and cells of transition ; the 

 first being those which do not proceed beyond the cell form in 

 their development ; the second, those which become trans- 

 formed into more permanent textures. 



1. Chyle and Lymph Cells. These are believed to be blood- 

 corpuscles in an early stage of development. They are for the 

 most part free nuclei, but a small number having cell-walls 

 around them. 



2. Blood- Cells. The blood-cells are either coloured or 

 colourless. The coloured cells are the ordinary blood-corpus- 

 cles of the blood. These cells have a character different in 

 fishes, reptiles, and birds from that which they exhibit in 

 mammals. In the three first-mentioned orders of vertebrated 

 animals, they are for the most part oval, being largest in 

 reptiles and smallest in birds. An oval vesicle or nucleus 

 occupies about a third of their area. Dilute acetic acid par- 

 tially dissolves the cell-wall and destroys the colour, while its 

 action is resisted by the nucleus. There is a fluid of a yel- 

 lowish tint between the nucleus and the cell-wall. In mam- 

 mals (except in the camelidse, in which they are oval), the 



