MODES AND CAUSES OF HUMAN TWINNING 127 



which Patterson has called ''primary budding," but 

 which I interpret as a very simple sort of bilateral fission 

 determined by the bilaterality of the uterus. If we 

 could only secure an early normal stage in man equivalent 

 to the first fission stage in the armadillo we could readily 

 settle the question as to whether the same mode of one- 

 egg twinning occurs in these two quite different mammals. 

 G. L. Streeter (1919) has recently made an exhaustive 

 study of a very early human one-egg twin embryo 

 (the Mateer ovum) which he thinks throws considerable 

 light on the question before us. This is much the 

 earliest stage of human twinning we have discovered and 

 deserves our careful consideration. 



The twin embryos are markedly different in size and 

 in stage of development. The larger one (which Streeter 

 calls the primary embryo) apparently lies in normal rela- 

 tion to the yolk sac and placenta (Fig. 49, C, p. 128). "It 

 is in the presomite stage and has only just acquired a prim- 

 itive groove." The smaller (which Streeter calls the twin 

 embryo) is in a stage about equivalent to. that of the 

 armadillo just prior to the first step of twinning or before 

 the embryonic axis is definitely established. This 

 smaller embryo is so abnormally situated with reference 

 to the larger embryo and to the placenta that it probably 

 never could have attained satisfactory nutritive relations. 

 It does not seem likely, therefore, that we have in this case 

 a typical instance of twinning such as might produce 

 duplicate twins. In addition to the fact that the smaller 

 twin is so obviously abnormal, the twins differ in other 

 ways from what must be the normal situation in duplicate 

 twins. Streeter's twins have entirely separate amnia, 

 and if the small twin were to placentate it would have a 



