ON EPIGNATHUS 21 



time when it still has the potency to form a whole embryo or part of 

 such, a parasitic malformation can originate. 



If at first the development of such a blastomere "sists" it becomes 

 " displaced," and may lie in various regions of the developing embryo. 



With progressive development there is a decrease of the potencies of 

 the several blastomeres. Hence Schwalbe argues that the more 

 complicated the structure of an epignathus, the earlier must be placed 

 the teratological process to which it owed its origin. 



Accepting the view that epignathus is an asymmetrical double 

 malformation, we have a certain amount of experimental work bearing 

 on the cause of the disturbance of normal development giving rise to 

 such. 



Thus there is evidence to show that, by the total separation of the 

 first two blastomeres of a single ovum, duplicate twins can result. 

 Incomplete separation of the first two blastomeres of a single ovum may 

 give rise to a symmetrical double malformation. All varieties of these 

 symmetrical double malformations result according to the extent of 

 separation and relative position of the blastomeres. 



In the production of an asymmetrical double malformation like 

 epignathus it may be that, as held by Wilder (^3), there has been a 

 complete separation of the first two blastomeres of a single ovum, and 

 then a fusion of the later blastomeres owing to close proximity. 



Possibly, however, such asymmetrical malformations are due to an 

 incomplete separation of a blastomere or blastomeres in a later stage of 

 development — perhaps at a stage when such blastomere or blastomeres 

 have not the full potentiality of a complete embryo. 



O. Schultze (21), in working with the eggs of amphibians, found that 

 among over-ripe eggs there was a larger proportion of double formations. 

 Over-ripe eggs tend to break up into non-nucleated pieces, and perhaps 

 these may become fertilised and produce a double formation. Boveri 

 has shown that a blastomere, either with or without a nucleus, can be 

 fertilised and develop. 



Polyspermia has been accepted by some as an origin of double 

 formations ; the researches of O. Hertwig (''^), Driesch, Fol and others 

 negative this. Besides, it has been shown that physiological polyspermia 

 takes place in many animals. 



The most probable view is that double formations originate after the 



(195) o I 



