192 The Study of Animal Life part hi 



poultry yard and in comparing the constitution of different 

 birds, that a highly-nourished and not "very energetic bird 

 will have larger eggs than one of more active habits and 

 sparser diet. 



The egg-shell consists almost wholly of carbonate of 

 lime, and the experiments of Irvine have shown that a hen can 

 form a carbonate of lime shell from other lime salts. It is 

 formed around the egg in the lower part of the oviduct, and 

 is often beautifully coloured with pigments allied to those of 

 blood and bile. These colours often harmonise well with 

 the surroundings, but how this advantageous result has 

 been wrought out is uncertain. 



Eggs differ greatly in regard to the amount of yolk which 

 they contain ; thus those of birds and reptiles have much, while 

 those of all mammals except the old-fashioned Monotremes 

 have hardly any. This is related partly to the number of 

 eggs which are produced, and partly to the amount of food- 

 capital which the embryo requires before other sources 

 of supply become available. The young of birds and 

 reptiles feed on the yolk until they are hatched, the unborn 

 young of all the higher (placental) mammals absorb food 

 from the mothers. The different sizes of egg usually 

 depend upon the amount of yolk, for the really vital portion 

 out of which the embryo is made is always very small. 



There are many differences also in regard to the outer 

 envelopes, witness the jelly around the spawn of frogs, the 

 firm but delicate skin around the ova of cuttlefish, the 

 "horny" mermaid's -purse enclosing the skate's egg, the 

 chitinous sheath surrounding the ova of many insects, the 

 calcareous shell in birds and most reptiles. 



5. The Male-Cell or Spermatozoon produced from a male 

 animal, or at least from a male organ (testis), is very differ- 

 ent from the ovum. It is very minute and very active. If 

 we compare an ovum to an Amoeba or to an encysted 

 Gregarine among Protozoa, we may liken the spermatozoon 

 to a minute monad Infusorian. It is a very small cell, 

 bearing at one end a "head," which consists mostly of 

 nucleus, prolonged at the other end into a mobile " tail," 

 which lashes the head along. 



