42 



HISTOLOGY. 



2d. Cells may elongate so as to form tubes or solid rods ; in the former 

 case they adhere by their ends to neighbouring cells, and their cavities 

 mutually open into each other, thus forming a vessel : in the latter the 



fluid content is lost, and a rod or 

 fibre is the result. Curious forms 

 are produced by a modification of 

 the same law, as exemplified in 



3d. Solid deposits may occur 

 within the cell wall, obliterating its 



L. The same thing may OCCIL 

 in the blastema, exterior to the cell 

 walls, and thus a solid will result. 

 Examples of the third and fourth 



kind occur in the formation of cartilage, as is illustrated in the accompa- 

 nying cuts. 



Fig. 6.f 



Fig. 7.f 



Fig. 8.f 



Fig. 9. 



Fig. 10. 



5th. A curious modification of development 

 occurs in the feathers of birds, where a nucleated 

 cell elongates and becomes filled with fibres ; the 

 cell wall is rubbed off by attrition, and the fibres 

 are thus uncovered and exposed. See fig. 10, 

 a, 6, c. 



Finally, it is believed by some that the blastema 

 may form a simple membrane or fibre without the 

 intervention of a cell, although this is by no means 

 proved. 



* Curious forms of cell transformation usually found in abnormal deposits. 



j- Development of cartilage. 



t Deposit in layers of lignin in the interior of vegetable cells. 



Mode of formation of the feathers of a bird in the interior of a nucleated celL 



