THE ANIMAL CELL. 39 



similar to that found in the ovum, and which is usually styled protoplasm ; and a 

 small spherical bodv imbedded in it, and termed a nucleus; the remaining con- 

 stituents of the ovum viz. its limiting membrane and the solid spot contained in 

 the nucleus, called the nucleolus are not considered essential to the cell, and in 

 fact many cells exist without them. 



Protoplasm (sarcode, blastema, germinal matter, or bioplasm) is a proteid com- 

 pound. It also contains certain inorganic substances, as phosphorus and calcium, 

 which latter appears to be essential to its life and function. It is of a semi-fluid, 

 vise-id consistence, and appears, sometimes, -either as a hyaline substance, homo- 

 geneous and clear, or as a granular substance, consisting of minute molecules 

 imbedded in a transparent matrix. These molecules are regarded by some as 

 adventitious material taken in from without, and often probably of a fatty nature, 

 since they are frequently soluble in ether. In most cells, however, protoplasm 

 shows a more definite structure, consisting of minute striae or fibrils arranged in a 

 clear transparent matrix, or a honeycombed reticulum containing in its interstices 

 a homogeneous substance. Protoplasm is insoluble in water, coagulates at 130 F., 

 and has a great affinity for certain staining reagents, as logwood or carmine. 



The most striking characteristics of protoplasm are its vital properties of 

 motion and nutrition. By motion is meant the power which protoplasm has of 

 changing its shape and position by some internal power in itself, which enables it 

 to thrust out from its main body an irregular process, into which the whole of the 

 protoplasmic substance is gradually drawn, so that the mass comes to occupy a 

 new position. This, on account of its resemblance to the movements observed in 

 the Amoeba or Proteus animalcule, has been termed "amoeboid movement." 

 Ciliary movement, or the vibration of hair-like processes from the surface of any 

 structure, may also be regarded as a variety of the motion with which protoplasm 

 is endowed. Nutrition is the power which protoplasm has of attracting to itself 

 the materials of growth from surrounding matter. When any foreign particle 

 comes in contact with the protoplasmic substance, it becomes incorporated in it 

 by being enwrapped by one or more processes projected from the parent mass 

 and enclosed by them. When thus taken up, it may remain in the substance of 

 the protoplasm for some time without change, or may be assimilated by the 

 protoplasm. 



The Nucleus is a minute body, imbedded in the protoplasm, and usually of a 

 spherical or oval form, its size having little relation to the size of the cell. It is 

 usually surrounded by a well-defined wall, the nuclear membrane, and its contents, 

 known as the nuclear substance, are composed of a stroma or network and an inter- 

 stitial substance, the relative amount of the two varying in different nuclei. The 

 network appears to be continuous through the nuclear membrane with the proto- 

 plasmic reticulum, from which it differs, however, in having strung along it bands 

 of a substance which stains readily with certain dyes, and is therefore named 

 chromatin. The chromatin differs chemically from ordinary protoplasm in con- 

 taining nuclein, in its power of resisting the action of acids and alkalies, in its 

 imbibing more intensely the stain of carmine, hsematoxylin, etc., and in its 

 remaining unstained by some reagents which color ordinary protoplasm ; as, for 

 example, nitrate of silver. 



The process of reproduction of cells commences in the nucleus, and is usually 

 described as being brought about by indirect or by direct division. Indirect division 

 or kari/okinesis (karyomitosis) has been observed in all the tissues generative cells, 

 epithelial tissue, connective tissue, muscular tissue, and nerve-tissue and it is the 

 typical method by which the division of cells takes place, although the process of 

 reproduction of cells by direct division occurs not infrequently, especially in highly 

 specialized cells. 



The process of reproduction by indirect division commences in the nucleus, the 

 stroma of which undergoes complex changes, leading to the division of this body 

 previous to the cleavage of the protoplasm of the cell. The changes consist briefly 

 of the following : (1) At the commencement of the process the nuclear network is 



