404 H. H. Newman and J. T. Patterson. 



In the case of the third five-embryo vesicle a satisfactory enum- 

 eration of the scutes was .not found possible, but the position of 

 he large disc was the same as in 108. 



In all three cases the amnia of the three embryos occurring on 

 the same side are irregularly arranged. Instead of occupying 

 whole quadrants of the subspherical vesicle the amnion of one or 

 more embryos is forced away from one end and crowded past the 

 opposite end, thus causing the amniotic partitions to run diago- 

 nally across the placental discs instead of taking a meridional 

 course from pole to pole as in typical cases. The relative positions 

 of the embryos is of course correspondingly irregular so that one 

 is immediately struck by it when the vesicle is first exposed to 

 view. 



The high degree of mal-adjustment seen in these vesicles would 

 seem to indicate that the occurrence of more than four embryos 

 is the expression of a coenogenetic tendency to carry polyembryony 

 a step farther by a doubling of the present typical number of 

 embryos. In the Mulita this condition has been attained and there 

 exists a strong tendency to double again, as seen in the frequency 

 of vesicles containing nine or more embryos. It appears probable 

 to us in view of the occurrence of one case of twins in our collection, 

 that in T. novemcinctum specific polyembryony had its origin in 

 the acquisition of a habit of producing identical twins in a fashion 

 similar to that seen in other mammals, that the inversion of germ 

 layers made it easy for this tendency to express itself still more 

 fully in the habitual production of four embryos. The production 

 of more than four embryos in our species seems to involve so 

 great a disturbance of a very accurate adjustment of embryos 

 and embryonic membranes that it seems highly improbable that 

 a larger number of embryos will ever become typical. 



It would be interesting to find out whether there is in T. hybri- 

 dum any tedency of the embryos to arrange themselves into two 

 groups corresponding to the right and left sides of the vesicle. A 

 study of Fernandez' photographs (figs. 1 and 2) would seem to in- 

 dicate that such is the case. It is hoped that this matter will 

 receive some attention and that the degree of resemblances 

 among the embryos of the various sets will be determined. 



