GERM CELLS AND THEIR FORMATION 97 



egg is fully formed an'd leaves the follicle, this process is with- 

 drawn, leaving a funnel-shaped canal. In a few instances 

 (some Insects) there are several micropylar perforations 

 through the egg membranes. Such openings are to be regarded 

 as specializations of the minute canals, mentioned above, which 

 give the appearance of the zona radiata to the membranes. 



Finally there is a great variety of tertiary membranes formed 

 by the walls of the oviducts, or by special glands in connection 

 with the reproductive system. These are applied outside the 

 chorion, or if this is absent, directly upon the vitelline mem- 

 brane. These envelopes may be of slime or jelly of an albu- 

 minous character, fibrous, or shelly coverings of chitin, lime, or 

 other substance. In forms depositing the eggs in the water this 

 is sometimes a thick jelly, holding the eggs together in strings 

 or masses, or serving to attach them, either singly or in masses 

 to plants, sticks, or other solid objects (e.g., Amphibia). These 

 tertiary membranes serve also, in special instances, as protec- 

 tion against drying, temperature changes, pressure, or mechani- 

 cal injury, and against the attacks of food-seeking organisms, 

 or infection by bacteria or other parasitic organisms. Fre- 

 quently they are nutritive in character, as the albumin or 

 " white" of the birds' eggs or the dense oily substance surround- 

 ing the eggs of the snails. 



Eggs may possess none or all three of these classes of mem- 

 branes ; sometimes only primary and secondary, or primary and 

 tertiary membranes are present. This usually depends upon 

 the nature of the egg-laying habits, method and duration of 

 development, and various other conditions. 



The spermatozoa, when fully formed, bear little resemblance 

 to ordinary cells, yet their individual history clearly shows them 

 to be such. In a few forms, chiefly among the Crustacea, the 

 sperms do resemble ordinary cells, and are often provided with 

 long radiating processes, sometimes, though rarely, pseudopo- 

 dial. But by far the most common form is that known as the 

 flagellate spermatozoon, found in all groups of animals from Pro- 

 tozoa to man. These are minute thread-like cells in which three 

 general regions can usually be made out (Fig. 51). One end is 



