10 HAND-BOOK OF PHYSIOLOGY. 



inal protoplasm of the embryonic blood-cells is replaced by the haemo- 

 globin of the mature colored blood-corpuscle. 



The minute structure of cells has lately been made the subject of care- 

 ful investigation, and what was once regarded as homogeneous proto- 

 plasm with a few scattered granules, has been stated to be an exceedingly 

 complex structure. In colorless blood-corpuscles, epithelial cells, con- 

 nective tissue corpuscles, nerve-cells, and many other varieties of cells, 

 an intracellular network of very fine fibrils, the meshes of which are 

 occupied by a hyaline interstitial substance, has been demonstrated 

 (Heitzmann's network) (Fig. 3). At the nodes, where the fibrils cross, 

 are little swellings, and these are the objects described as granules by 

 the older observers: but in some cells, e.g., colorless blood corpuscles, 

 there are real granules, which appear to be quite free and unconnected 

 with the intra-cellular network. 



(c. ) Nucleus. Nuclei (Fig. 3) were first pointed out in the year 1833, 

 by Robert Brown, who observed them in vegetable cells. They are either 



FIG. 3. (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 of newt showing intra-cellular network of fibrils (Heitzmann). Also 

 oval nucleus composeAf limiting-membrane and fine intra-nuclear network of fibrils. X 800. (Klein 

 and Noble Smith.) 



small transparent vesicular bodies containing one or more smaller particles 

 (nucleoli), or they are 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 "germinal matter." They exhibit their vitality by ini- 

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

 by first themselves dividing. Distinct observations have been made show- 

 ing that spontaneous changes of form may occur in nuclei as also in nu- 

 cleoli. 



Histologists have long recognized nuclei by two important charac- 

 ters: 



(1.) Their power of resisting the action of various acids and alkalies, 

 particularly acetic acid, by which their outline is more clearly defined, 

 and they are rendered more easily visible. This indicates some chemical 



