26 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



2. Eggs of the sea urchin. The eggs or ova of all animals are single cells. 

 Examine with the low power the slide marked "Arbacia mitosis" and note on 

 it the sections of the eggs of the sea urchin (Arbacia). With the aid of the assist- 

 ant find an unfertilized egg, recognized by its large, clear nucleus. Examine 

 under the high power. Note the very large nucleus, containing an unusually 

 large amount of nuclear sap, the chromatin granules in the nucleus, the large 

 black spot in the nucleus, called the nucleolus, or plasmosome, the delicate nuclear 

 membrane, and the cytoplasm packed with granules. The small black bodies 

 clinging to the periphery of the egg are the male elements, or spermatozoa, each 

 of which is also a single cell. 



Draw, showing all details. The granular appearance is best imitated by 

 stippling with the point of the pencil. 



B. STUDIES OF TISSUES 



The preceding study has been made on cells which are generalized in structure, 

 that is to say, cells which are like those found in very simple animals and which 

 have not become specialized. for the performance of particular functions. In an 

 adult organism so complex as the frog, however, there are very few cells which 

 retain this elementary structure, but most of them depart from the type plan 

 to a greater or less degree, depending upon the kind of function which they are 

 called upon to perform. Moreover, for the better performance of these functions, 

 the cells become united into orderly arrangements of layers or groups, and these 

 associations of cells are held together and aided in the performance of their func- 

 tions by certain materials, which they themselves secrete and which are called 

 intercellular substances. Such an association of a number of cells of a particular 

 kind with their particular kind of intercellular substance is called a tissue. We 

 shall now study the various kinds of tissues (Holmes, chap, vi, pp. 121-33). 



NOTE. In these studies of tissues, it is absolutely essential that the student 

 observe the following directions: (i) All material must be mounted in liquid, 

 either water (if dead) or physiological salt solution (if living). It is absolutely 

 useless to attempt to study dry material. The amount of liquid added should 

 be just sufficient to come to the edge of the cover glass. (2) All material must 

 be spread out as thin as possible, or picked into minute pieces with a pair of 

 teasing needles. It is absolutely useless and a waste of time to try to study 

 thick material. (3) After having made the material as fine as possible, put a 

 cover glass on, lowering it from one edge to avoid air bubbles. Excess fluid 

 should be absorbed. The cover glass should not float around on the material. 

 (4) Study the tissue first with the low power and without staining; then with the 

 high power. Draw what you can see in the unstained material. (5) Then 

 stain, if the directions say so, and add what you can see after staining to the 



