16 



PHYSIOLOGY OP THE DOMESTIC ANIMALS. 



{'. 



fe, 



;'';: 



wm % 



SB IPtil M 





Fig. 4.— Blood-Coepuscles 

 in Different Stages of 

 Development, showing 

 Reproduction by Fis- 

 sion. (Ranke.) 



(the blastema), forming the so-called " free cell-formation," or by the 

 subdivision of a previously-existing cell, — the " endogenous cell-forma- 

 tion." 



According to the first of these views, which may be compared to the 

 formation of crystals in a saline solution, granules first develop in a fluid 

 which contains all the chemical constituents of 

 the organism, forming the nucleolus of the 

 future cell. Around this other granules are 

 gradually deposited until the nucleus is formed, 

 and the cell-contents and membrane gradually 

 consolidate- around this. The first objection 

 to this theory, which, it is seen, implies spon- 

 taneous generation, lies in the fact that no one 

 has ever been able to demonstrate such a cell- 

 formation or to discover the so-called cyto- 

 blasts. It was then shown that all the cells of 

 the embryo originate in the segmentation 

 spheres of the ovum, and the falsit3 r of this doctrine of free cell- 

 formation is further proved from analogy by the manner in which the 

 connective-tissue cells take part in the development of pathological new 

 formations. There is now no more firmly-established dictum in plrysi- 

 ology than the statement that every cell originates from a previously- 

 existing cell. (Omnis cellula e cellida.) 



The other view, which was also to a certain extent advocated by 

 Schwann, as to the origin of cells by sub- 

 division of a parent cell, is exemplified in 

 the mode of reproduction of many of the 

 lower forms of life. Cells may reproduce 

 themselves by simple division of the parent 

 cell or by endogenous division. 



Cell reproduction always starts in the 

 nucleus. In simple division the nucleus first 

 becomes marked with a furrow, which grad- 

 ually deepens until the nucleus is divided 

 into two. The protoplasm of the cells is 

 then modified in the same way, until two new 

 cells are formed by the division of the 

 parent cell. This process may be followed in the reproduction of 

 the nucleated red blood-corpuscles of the embryo of the chick and 

 even in mammals (Fig. 4). A modification of this form of cell 

 reproduction is sometimes described as " budding.' 1 This process also 

 starts with the nucleus. A number of nuclei are first formed by the 

 subdivision of the nucleus ; these gradually separate ; the protoplasm 



Fig. 5.— Gemmation. A Bud- 

 dix« Germ-Cell of Gor- 

 dius, after meissner. 

 (Wundt.) 



