4 THE ESSENTIALS OF HISTOLOGY. 



that of the nucleus, and the intranuclear fibres undergo during its 

 division a series of remarkable changes which are known collectively 

 by the term karyokinesis (Schleicher). These changes may most easily 

 be studied in the division of epithelium-cells (see Lesson VI.), but 

 exactly similar phenomena have been shown to occur in cells belonging 

 to the other tissues. 



\. 



FlG. 4. TO ILLUSTRATE THE STRUCTURE OF CELLS AND NUCLEI. 



A, cell from the marrow ; p, protoplasm with fine reticulum ; n, nucleus, long and folded, 

 with intranuclear network. B, gland cell from a larva of Nemocera ; m, cell-membrane ; 

 p, protoplasm ; n, nucleus with convoluted filament, c, part of the nuclear filament in 

 B, greatly magnified, D, an amoaboid-cell (white blood-corpuscle) of the newt, very highly 

 magnified, showing a double nucleus with reticulum of chromoplasm, and the protoplasm 

 composed of two substances (spongioplasm and hyaloplasm). D is from a drawing by Mr. 

 D. Gunn ; A, B, and c are from Carnoy. 



In the early embryo the whole body is an agglomeration of cells. These have 

 all been formed from the ovum or egg-cell, which divides first into two cells, these 

 again into two, and so on until a large number of cells (embryonic cells) are pro- 

 duced. Eventually the resultant cells arrange themselves in the form of a mem- 

 brane (blastoderm) which is composed of three layers. These layers are known 

 respectively as the ectoderm or epiblast, the mesoderm or mesoblast, and the entoderm 

 or hypoUast. The ectoderm gives rise to most of the epithelial tissues and the 

 tissues of the nervous system ; the entoderm to the epithelium of the alimentary 

 canal (except the mouth), and the glands in connection with it; and the mesoderm 

 to the connective and muscular tissues. 



The tissues are formed either by changes which occur in the intercellular 

 substance, or by changes in the cells themselves ; frequently by both these pro- 



