236 THE BIOLOGY OF BIRDS 



are large eggs which develop into females, and small ones 

 which develop into males. Both develop without fertilisa- 

 tion, so that in these cases there is no question as to the 

 possible influence of the sperm. In one of the mites 

 (Pediculopsis) and in a primitive worm called Dinophilus, 

 in both of which fertilisation occurs as usual, there are large 

 egg-cells which develop into females, and small ones which 

 develop into males. Mere size means little, it is probably 

 an index of some constitutional difference in the rhythm of 

 the metabolism. 



Such cases as we have mentioned point to the idea that 

 there may be ab initio female-producing ova and male- 

 producing ova. This view is corroborated by the fact that 

 " identical twins," which develop from one ovum, are always 

 of the same sex ; and the same holds for the quadruplets 

 which one of the armadillos normally produces from one 

 ovum. They are all males or all females. In some of the 

 parasitic Hymenoptera, such as Encyrtus, one ovum forms 

 a group of embryos which are always of the same sex 

 — females if the egg be fertilised, males if it be not 

 fertilised. 



This leads to the suggestion that the decisive factor in 

 some cases may be the spermatozoon, and it is very significant 

 that in about thirty different kinds of animals there are two 

 kinds of spermatozoa, which differ in details of form, while 

 in many other cases, especially among insects and arachnids, 

 half of the spermatozoa have in their nucleus the same 

 number of nuclear rods or chromosomes as the ova have, 

 while the others have one fewer. When a spermatozoon with 

 the extra or X-chromosome fertilises an ovum which also 

 has the X-element, the result is a female. When a sperma- 

 tozoon without the X-chromosome fertilises an ovum which 

 has the X-element, the result is a male. It looks as if the 

 presence of two X-elements was necessary for the develop- 

 ment of femaleness, whereas the presence of one suffices for 

 the development of maleness. 



As regards birds there is considerable evidence that 

 there are two kinds of egg-cells, which differ in the rate or 



