194 CHARLES R. STOCKARD 



of oxygen becomes available. In exactly the same way the de- 

 velopment of the blastoderm in the fish's egg is experimentally 

 retarded or stopped by reducing the available oxygen and is again 

 made to resume its development by supplying oxygen. In the 

 case of the fish egg, the supply of ordinary nutriment is not in- 

 volved, and reactions similar to those of the armadillo egg are 

 only obtained as responses to changes in temperature or rate of 

 oxidation. 



I do not believe the retardation in the armadillo egg is of the 

 nature of a starvation phenomenon, since we see nothing of the 

 kind in other forms. Temperature changes are ruled out, since 

 the temperature of the uterus is more or less constant. The ab- 

 sence of oxygen necessary for the energetic process of cell divi- 

 sion is, therefore, in all probability the arresting cause, and the 

 retardation results in polyembryony. 



Thus Patterson has found the developmental interruption to 

 exist, and he has also shown the blastocyst to be disconnected 

 from the uterine wall and its necessary oxygen supply during this 

 time. However, he has furnished no data bearing on the reason 

 for the delay in uterine reaction and the consequent failure of 

 immediate implantation of the blastocyst such as normally occurs 

 in other mammals. 



The consideration of the armadillo egg up to this point has 

 taken account only of the external factors influencing its mode of 

 development. It must now be remembered as a fact of serious 

 importance that the production of quadruplets from the single 

 egg of the Texas armadillo is an almost constant occurrence, 

 while the experimental attempts to produce twins and double 

 individuals in fish eggs and other forms have given at best only 

 small percentages of such individuals among the large groups of 

 eggs treated. It is also recalled that all eggs do not furnish 

 equally favorable material for artificial twin production. The 

 eggs of the trout seem unquestionably more disposed to give rise 

 to twin formations than do the eggs of Fundulus. Thus some 

 eggs would seem to have an hereditary or truly innate predisposi- 

 tion toward polyembryonic formations. There is much reason 

 to believe that, aside from the external factors discussed, the 



