32 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



formed by the gray matter locate several very large cells. These are the motor 

 cells whose axones extend out to the voluntary muscles of the body. Examine 

 one of these with the high power. The structure of the cell body is well shown, 

 but only the beginnings of the processes are present. (Why?) Study the 

 structure of the cell body, comparing with other cells already seen. Note the 

 relatively large nucleus, containing a conspicuous round body, the nucleolus, 

 but very few chromatin granules, and the granular, sometimes fibrillar structure 

 of the cytoplasm. Draw. 



c) Structure of a nerve: Obtain a small piece of fresh nerve from a recently 

 pithed frog. Mount in salt solution and tease with needles in a longitudinal 

 direction until you have separated it out into minute fibers. Cover and examine. 

 The nerve is seen to be made up of a number of cylindrical fibers, bound together 

 by connective tissue. Each of the fibers is in fact a single axone of a nerve cell. 

 Find an isolated fiber and examine with the high power. It consists of a central 

 transparent region, the axone itself, often called here the axis cylinder, and a 

 thick sheath surrounding this, which is known as the medullary sheath. The 

 The sheath is composed of a fatty substance, called myelin, which tends to swell 

 up and become distorted in the preparation. Outside of the medullary sheath 

 is a delicate ceU wall, the neurilemma. Draw. 



6. Reproductive cells. 



a) Eggs: The eggs or ova of animals are the largest cells, although their 

 great size is generally due to the large amount of inert food material (yolk) 

 which they contain. The eggs of the frog are the spherical black and white 

 protuberances which have already been noted on the surface of the ovaries. 



6) Spermatozoa: The spermatozoa of the frog are best studied in the spring, 

 when the animals are sexually active, as at other seasons they are apt to be 

 non-motile and imperfectly developed. In such case, spermatozoa of a guinea- 

 pig or rat should be examined instead. Take a small portion of the testis of a 

 freshly killed frog or other animal, tease in salt solution, cover and examine 

 (Holmes, p. 216). The preparation will contain thousands of minute, slender, 

 rapidly moving objects, the spermatozoa. They are peculiarly modified cells. 

 In the frog each possesses a rodlike, slightly curved, head, which is practically 

 nothing but the nucleus, and a long, very slender, vibratile tail, which represents 

 the cytoplasm. The spermatozoa of other animals generally have smaller 

 rounded heads and longer tails than those of the frog. Draw a spermatozoon. 



