534 



Cleland (4), Dareste (5), Thomson (13) and Kaestner (8) but one 

 or two points have suggested themselves in dealing with the foregoing 

 cases. Briefly there are two rival theories put forward with regard 

 to such abnormalities ; firstly that they are due to the presence of two 

 germinal areas on one yolk which afterwards grow together to a 

 greater [or less extent and secondly that in the beginning there is 

 only one such area which at some stage undergoes partial or com- 

 plete fission. It is to the latter of these that the majority of writers 

 incline. Mitchell (9) in discussing the subject suggests that if the 

 plane of the first division of the ovum were at right angles to the 

 normal that there would be produced two cells each capable of pro- 

 ducing a right and left and so two embryos might result. He then 

 points out that from this theory it might well happen that the long 



-^. 



Figure 4. Photograph of 0-vum in ovo. 



axis of the monster would be parallel to the long axis of the egg, 

 but in the three cases just recorded and also in that recorded by 

 Spencer (loc. cit.) this is not the case. 



The recent cases of duplicity in the primitive streak stage re- 

 corded by TuR (14) and also that described by Burckhardt (3) seem 

 to point to the fact that although in a number of cases, possibly the 

 majority, the duplicity may be due to fission in the remainder it is 

 due to a fusion of two independent germinal areas. 



A full list of these abnormalities up to date was given by As- 

 sheton (1) in 1898, since which there have been the works of 

 Kaestner (1. c.) Bryce (2) and Tur (1. c.) all of which should be 

 referred to for a full discussion of the subject. 



