58 REPRODUCTION AND LIFE HISTORY OF ANIMALS. 



special glands or glandular cells in the walls of the oviducts 

 {e.g., the " shells " of many eggs). The envelope is often 

 firm, as in the chitinous coat around the eggs of many 

 Insects, and in these cases there is often a little aperture 

 (micropyle) through which alone the spermatozoon can enter. 

 The hard calcareous shells round the eggs of Birds and 

 Tortoises, or the mermaid's purse enclosing the egg of a skate 

 are of course formed after fertilisation. Egg shells must be 

 distinguished from egg capsules or cocoons, e.g., of the 

 earthworm, in which several eggs are wrapped up together. 



The Male Cell or Spermatozoon is a much smaller and 

 usually a much more active cell than the ovum. In its 



Fni. 8. — Forms of Spermatozoa (not (lra\\"n to scale). 



I and 2. Immature and mature spermatozoa of snail ; 3. of bird; 

 4. of man (//, head; w, middle portion ; ;", tail); 5. of salamander, 

 with vibratile fringe (X) ; 6. of Ascaris, slightly amoeboid with cap 

 (c) ; 7. of crayfish. 



minute size, locomotor energy, and persistent vitality, it 

 resembles a flagellate monad, while the ovum is comparable 

 to an amoeba or to one of the more encysted Protozoa. 



A spermatozoon has usually three distinct parts : the 

 essential " head," consisting mainly of nucleus, and the 

 mobile " tail" which is often fibrillated, and a small middle 

 portion between head and tail, which is regarded by some as 

 the centrosome. The spermatozoa of Threadworms and 

 Crustaceans are sluggish, and inclined to be amoeboid 

 (Fig. 8 (6, 7) ). 



Both ova and spermatozoa are true cells, and they are 



