36 



THE CELL 



a piece of masonry (Fig. 16) ; if a section be made of an egg, the 

 protoplasm is seen to be present in the form of a delicate net- 

 work, in the larger and smaller meshes of which these i-eserve 

 substances are deposited. The only place where it is collected 

 together into a thick, cohesive layer is on the surface of the egg, 

 and in the neighbourhood of the nucleus. 



Another good example of a protoplasmic framework structure, 

 caused by the deposition of various substances, is afforded us by the 

 mucous cells of vertebrates (Fig. 17) and invertebrates. The 

 section varies according as to whether it is taken from the 

 epithelial surface, or from the base of the goblet. In the former 

 case it is wider, and is seen to consist chiefly of homogeneous 

 shining secretion, the mucilaginous substance, which is evacuated 



FIG. 16. An egg of Ascaris megalocephala, FIG. 17. Goblet-cell from the 



which has just been fertilised (after Van Bene- bladder epithelium of Squatina vul- 



den ; from O. Hertwig, Fig. 22) : sfc spermato- garis, hardened in Muller's fluid, 



zoon, with its nucleus which has just entered ; (After List, Plate I., Fig. 9.) 

 / glistening fatty material of spermatozoon: 

 fcb female pronucleus. 



from time to time by the cell, through a small opening at its free 

 end, and transformed into mucin. The protoplasm traverses the 

 mass of secretion in the form of fine threads, which join together 

 to make a wide meshed network, only forming a compact body at 

 the lower extremity of the cell, in which also the nucleus is 

 situated. 



II. The Chemico-physical and Morphological Properties 



of the Nucleus. The nucleus is quite as important as the 



| protoplasm in the economy of the cell. It was first discovered, 



! in 1833, by Robert Brown (I. 5), in plant-cells ; soon afterwards 



( Schleiden (I. 28) and Schwann (I. 31) made it the foundation 



stone of their theory of cell formation ; after that the study of 



the nucleus remained for some time in the background, as the 



