42 NORMAL HISTOLOGY. 



two cells of the same sex. It is illustrated among aphides and cer- 

 tain of the humbler lepidoptera. 



(c) Fertilization. This occurs when two sexual cells unite to 

 produce a new individual. It is a process common to higher plants 

 and animals. 



It is evident, therefore, that each individual plant or animal has 

 its origin in a single cell, and that its organism is developed by a 

 process of cell-multiplication. For example, by the fusion of the 

 sexual elements the primordial cell of the animal body is formed. 

 This cell divides by a process of karyokinesis and forms two cells. 

 Each of these divides, similarly, forming four cells; the four like- 

 wise produce eight ; and the eight produce sixteen. This gives what 

 is called the morula, or mulberry stage. By rearrangement of these 

 cells in the form of a pouch there is formed the gastrula, the cells 

 disposing themselves in two layers. From these layers is produced 

 a middle layer, thus forming the blastoderm, consisting of three 

 distinct layers, epiblast mesoblast, and hypoblast. From these 

 layers, by cell-multiplication, growth and differentiation, all the 

 structures of the body are produced. 



Laboratory exercise No. 4.. The structure of a cell. Peel from the 

 outer surface of a scale of an onion bulb a piece of the epidermis. Ap- 

 ply to a slide and stain for a few moments with rosanilin violet. Wash 

 with water, apply cover-glass, and examine. Observe first the form of 

 the cells and their relative positions, then the structure of each cell. 

 Make out the cell-wall, the nucleus with its nucleolus, and the gran- 

 ular protoplasm surrounding it. Do you observe any vacuoles? Make 

 a drawing of several cells, exhibiting the structures above named. 



Laboratory exercise No. 5. To demonstrate protoplasm and cellulose. 

 Make a preparation similar to the above and apply a drop of iodine 

 solution. Examine. The protoplasm will be stained brown, but the 

 cell-wall is not stained. Now remove cover-glass and apply a small 

 drop of sulphuric acid. This changes the cellulose into soluble dex- 

 trin, which is attacked by the iodine and turned blue or black. This 

 is the iodine test for starch. Look for crystals of iodine and for the 

 Brownian movement among the molecular particles. 



THE BROWNIAN MOVEMENT. 



This is a molecular movement purely physical in character. It 

 occurs amoiur bacteria and may be mistaken for independent mo- 

 tion due to vitality. 



