196 CHARLES R. STOCKARD 



stage, it might still be capable, on resuming development, of 

 giving a normal single embryo. The egg of the deer may possess 

 only a very slight tendency toward accessory embryo formations. 

 A study of the experimental production of twin and double indi- 

 viduals among fish leads one to be surprised at the case of the 

 armadillo and to expect the reaction found in the deer. The 

 constant interruption occurring in the development of the birds 

 and other animals at indifferent developmental moments with 

 no subsequent ill effects renders commonplace the fact that the 

 deer successfully withstands an interruption during its develop- 

 ment without noticeable modifications in structural response. 



In conclusion we may summarize the cases as follows. The 

 development of the armadillo is interrupted on account of a 

 failure to become promptly implanted on the uterus and a con- 

 sequent exhaustion of available oxygen supply. The interrup- 

 tion occurs at a critical period just preceding the primitive- 

 streak and embryonic-line formation. The internal qualities of 

 this egg gives to it a decided tendency under conditions of arrest 

 to form accessory embryonic buds. As a result of the inter- 

 action of these external and internal forces polyembryony is 

 prpduced. 



In the case of the deer only one probable fact is known, and 

 that is that a 'period of quiescence' occurs. It is uncertain at 

 what stage the arrest takes place, but it is probably due, as in 

 the armadillo, to a delayed implantation of the blastocyst. Either 

 on account of the stage of arrest or a lack of tendency to form 

 accessory embryo buds, a typically single individual arises 

 from this egg. The external factors may be the same as in the 

 case of the armadillo, but they interact with different internal 

 factors or different developmental moments to give a very differ- 

 ent result. 



