no THE PHYSIOLOGY OF TWINNING 



which must be thought of as homologous with the 

 medullary plate rolled up into a hollow ball, has thinned 

 out in the roof to form the ectodermal layer of the 

 amnion. The center of the floor is the region where 

 we would expect to see the first sign of a differentiation 

 of the apical point of the new axis; but this part is even 

 a little thinner than are the lateral walls, directly to 

 the right and to the left of the original apical region. 

 The only evidence of a definite bilaterality in the ecto- 

 dermic vesicle is seen in the fact that it is broader in one 

 plane than in any other. The figure is drawn in this 

 plane and shows in the mesoderm further evidences 

 either that bilaterality has persisted in the vesicle or 

 else that it has been established de novo as a result of the 

 position of the vesicle with respect to the axis of the 

 uterus; for the bilateral axis of the embryo coincides 

 with that of the uterus even at this early period. The 

 mesoderm begins to proliferate from two points where the 

 ectoderm and endoderm part company. For some time 

 the mesoderm consists of a considerable number of 

 isolated thin-walled vesicles, but there is always a period 

 when these small vesicles break together into two large 

 vesicles, separated by a median mesentery that coincides 

 with the principal axis of the untwinned embryo. Con- 

 cerning this point it is important to note Patterson^s 

 comment: 



The earliest observed evidence which could be interpreted 

 as representing the beginning of multiple embryos comes in the 

 formation of the mesothelium — not in the manner in which the 

 elements of this layer arise, for localized centers of proHferation 

 were not found, but in the early formation of two large mesodermal 

 vesicles through the fusioi\<of smaller ones. The development of 

 two mesodermal vesicles would not in itself be so significant, as it 



