62 THE BIOLOGY OF TWINS 



each one has filled a full quadrant of the vesicle and 

 has fused with adjacent amnia wherever contact has 

 been estabhshed. Why the pushing over of the amnia 

 always goes toward the left (anti-clockwise) and never 

 to the right (clockwise) is a problem in developmental 

 mechanics for which I have no solution. Evidently, 

 however, the vesicle is still acting as a unit and respond- 

 ing to the same predetermined bias toward the left which 

 was expressed in the period of embryonic segregation 

 when each primary individual always pairs with a sec- 

 ondary individual at its left side.' 



An egg at full term has a transparent area at both 

 proximal and distal ends and the broad but broken 

 placental zone about the equatorial region. It is 

 readily seen that, when the arrangement of fetuses is 

 so diagrammatic, there can be no difficulty in preserv- 

 ing their paired arrangement. One has merely to open 

 the vesicle at the point {v n v) in each case in order 

 to be able at any subsequent time to identify fetuses 

 I, II, III, and IV. This situation is, of course, highly 

 favorable for the study of correlation and heredity, as 

 will presently appear. 



Atypical numbers of fetuses in D. novemcinctus . — The 

 description of embryonic development herewith pre- 

 sented appUes to a large proportion of cases. In about 

 3 per cent of cases, however, the diagrammatic relations 

 typical for the species are distorted by the development 

 of more or less than the typical number of embryos. 



^ It is only natural to refer in this connection to the asymmetry 

 present in the maturating ovocyte (Fig. 4), where the maturation 

 spindle is "seen to occur at one side of the formative zone. There may 

 be established here the basis of a growth bias toward the left. 



