MODES AND CAUSES OF HUMAN TWINNING 133 



slightly before the other and developed into the single living girl. 

 The second egg was not so favorably implanted as the first; this 

 is indicated in the specimen by the lower placenta riding upon the 

 larger one. The delay in implantation, due to the presence of the 

 first egg, caused a slow rate of development at an early stage in 

 the second and two embryonic buds arose instead of one, just as 

 was described on the germ ring of the fish. In this human specimen 

 there is fortunately present the physical cause that might have 

 produced the delay. 



This explanation of Stockard's has unfortunately a 

 very limited application, for it is extremely rare that one- 

 egg twins occur along with another embryo. As a rule 

 the egg destined to produce twins has the whole uterus 

 to itself; it could not be retarded by the prior placenta- 

 tion of another egg. We must therefore look elsewhere 

 for probable retarding agencies. Three possible retard- 

 ing factors seem possible : 



1. Under stimulation of the egg, due to some defect in 

 the development-initiating mechanism of the sperm. Dav- 

 enport has shown that twinning is rather strongly inher- 

 ited in the male line. If this be the case it could hardly 

 affect two-egg twinning, since this is a phenomenon of 

 ovulation and concerns only the female. It would 

 seem then that only one-egg twinning could be affected 

 through the male line. If the egg were retarded through 

 insufficient stimulation on the part of the sperm it would 

 probably undergo belated fission, the consequences of 

 which would depend upon the degree of retardation. 



2. Belated placentation, due to a failure of the corpus 

 luteum to stimulate the uterine mucosa. This condition 

 merely implies some physiological discoordination be- 

 tween the various intricately interdependent factors 

 responsible for implantation of the ovum. The weakness 



