258 THE BIOLOGY OF BIRDS 



Probably, therefore, it is advantageous that the egg-cell 

 should rapidly become non-receptive when one spermatozoon 

 has entered. 



{e) But fertilisation must imply something more, for it 

 is followed by segmentation or cleavage, which does not 

 usually occur in the absence of a fertilising spermatozoon. 

 Whether it is that the spermatozoon introduces some positive 

 stimulus, such as a ferment, activating the nucleus of the egg 

 so that division sets in ; or whether it is that the entrance 

 of the spermatozoon removes or counteracts some inhibiting 

 influence which was keeping the ovum from beginning to 

 divide ; it would be rash at present to say. It may be noted 

 that the researches of Delage, Loeb, and others have shown 

 that it is possible in a variety of ways to induce aspermic 

 (or artificially parthenogenetic) development in eggs, but this 

 has not yet succeeded above the level of frogs. 



There have been various reports of futile attempts at 

 parthenogenetic development in the eggs of fowls. Thus 

 A. Lecaillon (1909) observed in unfertiUsed eggs of the fowl 

 the formation of a disc of cells, taking place by slow division 

 of the usual (mitotic) type. The appearance of the cells 

 was abnormal, and all of them underwent degeneration. 



Artificial insemination of hens and pheasants has been 

 effected (E. Ivanow, 1913), and a small percentage of the hens 

 laid fertile eggs which developed. 



§ 6. The Further History of the Fertilised Ovum 



When the ovum is liberated from the ovary it is caught 

 by the adjacent trumpet-shaped mouth of the oviduct. 

 Fertilised either at the top of the oviduct or previously, it 

 begins to pass slowly downwards, developing as it does so, 

 and undergoing a succession of interesting changes. It is 

 surrounded by various instalments of white-of-egg or 

 albumen ; it is surrounded by a shell-membrane and a 

 calcareous shell ; the shell is usually stained with pigments 

 before it is finally set ; it is moved onwards, broad end 

 foremost, and is finally expelled through the cloaca by the 



