256 THE BIOLOGY OF BIRDS 



The spermatozoa of birds are typical, showing (a) a 

 " head " mostly consisting of the chromosomes, which are 

 certainly the vehicles of at least a great part of the hereditary 

 qualities (for there is a minimum of extra-nuclear cytoplasm) 

 (b) a minute " middle piece " bearing a " centrosome," 

 which afterwards plays an important part in the division of 

 the fertilised egg-cell ; and (c) a locomotor " tail " which is 

 left outside when the spermatozoon enters the ovum. 



It is in the nature of typical spermatozoa, little more can 

 be said, to move against a current. This can be demon- 

 strated experimentally under the microscope. Now as 

 there is a slight downward current of secretion in the oviduct 

 of the female bird, the spermatozoa will tend to meet the ova. 

 This meeting may be at the top of the oviduct. In some 

 cases the meeting is believed to be in the ovary itself. What 

 does the fertilisation imply ? 



§ 5. Fertilisation 



Fertilisation may be defined as the intimate orderly 

 union of two sex-cells, the spermatozoon and the ovum. It 

 is a microscopic business ; the transference of spermatozoa 

 from the male to the female should be designated by a 

 different term — insemination. 



(a) Fertilisation implies the mingling of two inheritances, 

 or of the " factors " of two inheritances, the paternal and the 

 maternal. These " factors " or " genes " or determinants 

 of hereditary qualities are carried in great part by the chromo- 

 somes, which are equal in number in ripe ovum and ripe 

 spermatozoon. It may be, however, that some of them are 

 carried in the extra-nuclear cytoplasm or cell-substance of 

 the germ-cell. Whether this be so or not, it is certain that 

 the extra-nuclear cytoplasm is very important in furnishing 

 initial building material. But as the amount of cell-substance 

 or cytoplasm, as distinguished from chromosomes or nucleo- 

 plasm, is infinitely minute in the spermatozoon compared 

 with the amount in the ovum (the formative droplet already 

 alluded to), it seems impossible to avoid the conclusion that 



