A TEXT-BOOK OF EMBRYOLOGY 



CHAPTER I. 



THE CELL AND CELL PROLIFERATION. 



THE CELL. 



The Typical Animal Cell (Fig. i) is a small definitely restricted mass of 

 protoplasm. It contains or has at some period of its development contained 

 two specially differentiated bodies, the nucleus and the centrosome. It may be 

 limited by a more or less definite cell membrane. 



Of the ultimate structure of living protoplasm our knowledge is extremely 

 small. It is of an albuminous nature, coagulated by heat and by many chemical 

 reagents. It varies both in structure and in chemical composition in different 

 cells and is probably best considered, not as a definite structure either chemically 

 or morphologically, but as the material basis of life activities. Protoplasm can 

 usually be resolved into a formed part, spongioplasm, which takes the form of a 

 reticulum, a feltwork, or fibrillar, and an unformed homogeneous element, 

 hyaloplasm, which fills in the meshes of the reticulum or forms the perifibrillar 

 substance. Various protoplasmic inclusions are frequently found in cells. To 

 these the term metaplasm (paraplasm, deutoplasm) has been applied. Among 

 them may be mentioned plastids, fat droplets, pigment granules and various 

 excretory and secretory substances. 



The Nucleus is usually separated from the rest of the protoplasm by a 

 nuclear membrane. Within the nucleus the nuclear membrane is continuous 

 with a nuclear reticulum which consists of two parts: a chromatic part — chroma- 

 *tin, and an achromatic part — linin. At nodal points of the network there are 

 frequently considerable accumulations of chromatin to form net knots (false 

 nucleoli or karyosomes). Filling the meshes of the nuclear reticulum is a fluid 

 or semifluid substance, the nucleoplasm or karyoplasm. The structure of the 

 nucleus is thus seen to correspond closely to the structure of the surrounding 

 protoplasm. This is especially evident in those cells in which there is no 

 limiting nuclear membrane, the nuclear reticulum and the cytoreticulum being 

 continuous, the nucleoplasm and cytoplasm mingling. This condition, true 



