2 ANIMAL CELL 



even under high powers of the microscope, no signs of any finer structure ; in 

 other cases, especially after fixation, it exhibits a meshwork or reticular 

 appearance, which has been variously interpreted as indicating a filar, a spongy, 

 or an alveolar structure, but in view of the effects produced by most fixing 

 reagents on colloid solutions, such interpretations must be received with caution. 



The granules which are generally present in the cytoplasm may be either 

 essential constituents of the protoplasm, or included non-protoplasmic bodies of 

 various kinds. The granules of the former kind (cytomicrosomes) are very minute, 

 and form apparently an important element in the active protoplasm. They have 

 been called mitochondria by Benda, and have been shown, as we shall see, to play 

 an important part in the sex-cells. The non-protoplasmic granules include such 

 as pigment-granules, yolk-grains, and so on ; but larger inclusions, such as fluid 

 vacuoles, fat-drops, &c., also occur. All these may be collectively designated 

 by the terms deutoplasm or paraplasm. 



While we generally speak of the tissues as being composed of separate cells, we shall have 

 occasion in the course of this work to refer to instances of tissues in which the so-called cells 



FIG. 2. POLYMORPHONUCLEAB LEUCOCYTE OF 

 LEPIDOSIBEN, SHOWING LOBED NUCLEUS, 



FIG. 1. LEUCOCYTE (Lepidosiren paradoxa) ATTRACTION - SPHERE, AND SOME GBA- 



SHOWING ATTBACTION-SPHEBE. (T. H.Bryce.) NULES. (T. H. Bryce.) 



are joined together by protoplasmic strands. The tissue in such cases is really a multinucleated 

 protoplasmic network. In other instances, owing to the division of the nuclei without cleavage 

 of the protoplasm, a multinucleated mass or layer of protoplasm is produced. Any such 

 multinucleated mass is termed a syncytium. 



Centrosome s central particle. In most cells there is generally to be 

 demonstrated a point in the cytoplasm, as a rule close to the nucleus, where by 

 suitable stains a single granule, a double granule, or a group of granules, may be 

 made visible. These are surrounded by a clear structureless area, round which 

 the protoplasm may be arranged in a radial fashion. When fully developed, as 

 in wandering leucocytes (figs. 1 and 2), the whole arrangement is named the 

 attraction sphere. The substance of the sphere is known as the archoplasm or 

 centroplasm. The central area, containing the granule or granules, has been 

 named the centrosome by Boveri. When it contains a single granule, the 

 particle is called the centriole. In cells in which the granule is the only obvious 

 feature, it may itself be termed the centrosome. A centrosome is absent in plant- 

 cells, and it has been proved by Morgan and Wilson that focal points, having 

 all the characters of centrosomes, may be produced in the protoplasm of the 

 echinoderm-egg by treatment with chemical reagents. 



Nucleus. The nucleus in the vast majority of cells is a spheroidal or slightly 

 ovoidal body ; but it may be lobed as in leucocytes (figs. 1 and 2). It has a definite 



