EGGS AND ANTHEROZOIDS 183 



The eggs of the birds before they are released from the ovary 

 are amongst the largest cells we know. The eg(r of the ostricii 

 is from 150-155 mm. long and 110-130 mm. wide. It weighs 

 1440 gms. and is equal to some 24 fowls' eggs. But the extinct 

 Aepyornis, of Madagascar, several of whose eggs are known, 

 surpasses even this, being about 340 mm. long and 220 mm. 

 wide. It has a capacity of 3-7 litres, and equals at least 

 twelve ostrich eggs or 288 average domestic fowls' eggs. It 

 is 500,000 times bigger than the small egg of the humming- 

 bird. The curious New Zealand kiwi, Apteryx, •has an egg 

 quite out of proportion to the size of the bird that lays it. 

 The egg weighs almost a quarter of the body-weight. Of 

 course it must be remembered that these eggs consist almost 

 entirely of yolk, but the amount of metabolism that must go 

 on in the bird's body to produce such an ovum must be 

 tremendous. The actual protoplasmic egg is minute. 



Occasionally ova are amoeboid. This is true, as we have 

 seen, of the Hydra. Very often the egg is oval or sausage- 

 shaped. This is frequently the case in insects, where the egg 

 case may be a structure of great symmetry and beauty, 

 resembling in some cases seeds and in others little chalices 

 with a cross on the top. Sometimes the egg case has a special 

 aperture or micropyle for the admission of the spermatozoa. 

 More often the spermatozoa may penetrate at any part of 

 the surface and, as a rule, when once a spermatozoon has 

 penetrated, a membrane is formed which prevents the access 

 of others ; but this is by no means a universal rule and some 

 eggs, such as those of the dogfish, receive many spermatozoa. 



Antherozoids and Spermatozoa 



As a rule the male gametes of plants move by means of two 

 flagella. Some antherozoids have regular tufts or bunches of 

 cilia, as in the adder's-tongue and other ferns. The two 

 flagella may arise at one end of an oval body or they may 

 emera"e from the side, one stretching forward and one back- 

 ward. Antherozoids have been modified in the fhnvering 

 plants into two naked cells which appear in the pollcn-tu[)e 

 and one of which fuses with a similar naked cell in the 



