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HANDBOOK OF PHYSIOLOGY. 



(d.) According to their functions, they are called secreting, protec- 

 tive, sensitive, contractile, etc. 



(e.) According to their origin, they are named epiblastic, mesoblastic 

 and hypoblastic. 



Nearly all cells at some period of their existence possess nuclei. As 

 has been incidentally suggested, the origin of a nucleus in a cell is the 



\ 



FIG. 11. Various forms of cells, a. Cylindrical or columnar ; 6. Caudate ; c. Fusiform ; A 

 Ciliated (from trachea) ; e. Branched, stellate. 



first trace of the differentiation of protoplasm. The existence of nuclei 

 was first pointed out in the year 1833 by Robert Brown, who observed 

 them in vegetable cells. They are either small transparent vesicular 

 bodies containing one or more smaller particles (nucleoli), or they are 



FIG. 12. (A. ^ Colorless blood-corpuscle showing intra-cellular network of Heitzmann, and two 

 nuclei with intra nuclear network (Klein and Noble Smith'. (B ) Colored blood-corpuscle showing 

 intra-cellular network of fibrils (Heitzmann). Also oval nucleus composed of limiting membrane 

 and fine intra-nuclear network of fibrils, x 800. vKlein and Noble Smith.) 



semi-solid masses of protoplasm always in the resting condition bounded 

 by a well-defined envelope. In their relation to the life of the cell they 

 are certainly hardly second in importance to the protoplasm itself, and 

 thus Beale is fully justified in comprising both under the term " germi- 

 nal matter." They exhibit their vitality by initiating, in the majority 

 of cases, the process of division of the cell into two or more cells (fission) 

 by first themselves dividing. Distinct observations have been made, 

 showing that spontaneous changes of form may occur in nuclei as also in 

 nucleoli. 



