ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 45 



arise from the transformation of a greater quantity of protoplasm, 

 and almost the whole contents of the cell are concerned in their 

 production. Rarely the cells remain single, and never acquire more 

 than one nucleus, so that the muscle is composed of only a single cell 

 (eye muscles of Daphnia). Sometimes the cells become elongated 

 into long tibres, the primitive bundles ; the nuclei at the same time 

 increase in number, and a membrane, the sarcolemma, becomes 

 developed on the outer surface of each fibre. More frequently, 

 however, the primitive bundles arise by the fusion of several cells 

 placed in a row. Either the nuclei come to lie close to the sarco- 

 lemma in a peripherally-placed layer of finely granular protoplasm, 

 or they are arranged in a row in the axis of the fibre in some finely 

 granular non-contractile protoplasm. The finer and coarser muscular 

 bundles are composed of many primitive bundles (fibres) placed close 

 together and held together by connective tissue. The fibrillation of 

 the muscular bundles corresponds to the direction of the primitive 

 bundles (muscles of Vertebrata). Finally, both the simple cells, and 

 the multi-nucleated muscles which arise from them, may be branched 

 (heart of Vertebrata, intestine of Arthropods, etc). 



4. Nervous tissue. As a rule, nervous tissue is found with mus- 

 cular tissue, and is the means by which stimuli are conveyed to the 

 latter; but above all, it is the seat of sensation and the will. "With 

 regard to this important function it would appear probable that in 

 phylogeny the elements of nervous tissue have not arisen in con- 

 nection with muscular tissue, but in connection with the sense 

 cells found in the skin, i.e., differentiated ectoderm cells, and that 

 then, still remaining connected with the sense-cells, they have 

 travelled inwards into the subjacent tissue ; while the connection 

 with the muscle-cells, which at first possessed an independent 

 irritability, is only secondary. 



Nerve-tissue contains two distinct structural elements, nerve cells 

 or ganglion cells, and nerve fibres ; both possess a distinct minute 

 structure and molecular arrangement, as well as chemical compo- 

 sition. 



The ganglion cells act as centres for nerve-stimuli, and are found 

 especially in the central organs which are known as brain, spinal 

 cord, or simply ganglia. They usually possess a finely granular 

 contents, with a large nucleus and nucleolus and one or more pro- 

 cesses (unipolar, bipolar, multipolar, ganglion cells), one of which 

 is the root of a nerve fibre (fig. 37, a, b). 



Frequently the ganglion cells are enclosed in connective tissue 



