218 PHYSIOLOGY AT THE FARM. 



as blood-vessels, nerve, tubes, fibres, membranes, bone, and 

 cartilage. 



Laws which regulate the Origin and Development of Cells. 

 The laws which regulate the origin and development of cells 

 afford some insight into the plan on which organic bodies grow 

 and are sustained. The fluid in which cells originate is termed 

 blastema or germ-substance. At first this fluid is clear, and 

 after a time it becomes opaque by the formation in it of numer- 

 ous molecules or granules. Such a fluid in every known instance 

 owes its origin to a previously existing living organism. 

 The molecules and granules coalesce and give rise to a larger 

 body, on which a delicate membrane is produced and gradually 

 detached therefrom by the accumulation of fluid. The cell so 

 developed, on attaining perfection, consists of an external 

 envelope or vesicle, termed the cell- wall ; the body within 

 which the cell-wall was produced, now termed the nucleus ; and 

 a fluid interposed between the inner surface of the cell-wall and 

 the outer surface of the nucleus. In the nucleus may often 

 be remarked one or two included granules ; granules of this 

 character are named nucleoli. 



Cells exhibit varied phenomena : they may gradually dis- 

 solve and perish, or, the cell-wall bursting, the fluid contents 

 may be set free, constituting a secretion various matters may 

 be deposited in the interior of the cell, such as pigmentary 

 substance, a mineral matter, bone, and the like the cells may 

 unite with contiguous cells to form complex tissues, as fibres, 

 tubes, networks. Cells, again, often show a reproductive pro- 

 perty: thus, the cell-wall may bursf and set free included 

 germs, each of which shall give origin to a new cell ; a nucleus 

 may enlarge and divide into two, and these, again, into other 

 two, the multiplication here being within the original cell-wall ; 

 the cells may resolve themselves into two or more divisions ; or 



