xiv THE CONTINUANCE OF LIFE 307 



lies 011 the top of the yellow yolk and has a nucleus for 

 its centre ; round about there is a coating of white-of-egg ; 

 this is surrounded by a double membrane which forms 

 an air-chamber at the broad end of the egg ; outermost 

 is the porous shell of lime. 



The cell-substance of the ovum, when it is not en- 

 cumbered or disguised with a large or relatively large 

 quantity of yolk, sometimes shows an intricate proto- 

 plasmic structure. Of great interest is the occasionally 

 demonstrable presence of definite formed bodies (mito- 

 chondria, etc.) in the protoplasm, which in some instances 

 have been proved to be "organ-forming substances." 

 For if they are carefully removed, the embryo is lacking 

 in certain parts. Within the nucleus (or germinal vesicle) 

 of the ovum there are readily stainable chromatin-ele- 

 ments or chromosomes, always definite in number. 



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 

 mother. 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. 



6. The Male-Cell or Spermatozoon produced from a 

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

 different from the ovum. It is very minute and very 

 active. If we compare an ovum to an Amceba or to an 

 encysted Gregarine among Protozoa, we may liken the 



