66 THE BIOLOGY OF TWINS 



One might have been led to suspect the occurrence 

 of exceptions to the rule that the embryos of D. novem- 

 cinctus consist of two pairs, one derived from the right 

 and one from the left primary ectodermic outgrowth, 

 for, long before the information just given was avail- 

 able, it was known that the inter-resemblances of quad- 

 ruplet sets were occasionally out of accord with the 

 paired arrangement. Sometimes three out of four 

 fetuses were alike in the possession of some one genetic 

 feature and the fourth was different; similarly, it was 

 occasionally noted that but one fetus possessed a 

 peculiarity and the other three were without it. At 

 the risk of anticipating the conclusions expressed in a 

 subsequent chapter I may say that, were we in posses- 

 sion of the facts as to the origin of the four quadruplets 

 from the common ectodermic vesicle, all of the data 

 on resemblances, which are to receive treatment sub- 

 sequently, would be completely rationalized. 



DEVELOPMENT OF THE PLACENTA 



The history of the placenta is of considerable impor- 

 tance for an understanding of twinning. It gives us 

 the criteria for distinguishing the four embryos and 

 their paired or non-paired relationships even in the 

 last stage of uterine development. A brief resume 

 of the facts relating to the placenta will accordingly 

 insure a clearer understanding of what is to come. 

 The primary placenta is the Trager, a ring of specialized 

 trophoblast that forms the first connection with the 

 uterine mucosa. This Trager subsequently, as in 

 Fig. 17, becomes uniformly studded with adhesion 

 pads destined to become the burrowing tips of the 



