io8 



COMPARATIVE ANATOMY 



have been formed by the activity of the cell-protoplasm. 

 Fatty or adipose tissue appears under the microscope as a 

 number of minute oval vesicles, generally gathered into a 

 cluster called a lobule, which receives an afferent artery and 

 an efferent vein. Each vesicle is filled with an oily matter 

 which can be dissolved out by the action of ether, and it can 

 then be seen that the vesicle consists of a very delicate 

 structureless membrane, on one side of which is a flattened 

 nucleus, surrounded by a few granules of protoplasm. In some 



Fig. 22. 



Part of a fat lobule from the mesentery of a rabbit, highly magnified, to 

 show the structure of adipose tissue. (From a drawing by Dr 

 E. H. Schuster.) 



of the vesicles a wide-meshed reticulum of delicate proto- 

 plasmic strands may be seen, in the cavities of which the 

 oil-drops were lodged. A fat-cell is a good example of the 

 endoplastic formation of material by a living cell. 



We have now studied the microscopical characters of the 

 tissues sufficiently to enable us to comprehend the fact that 

 the whole organism is composed of a number of minute living 

 units called cells, which, indeed, are seldom to be found in a 

 primitive and undifferentiated condition in the adult animal, 

 but are variously modified, changed, and adapted to particular 

 purposes, and consequently exhibit every variety of form and 



