THE AMEBIC AN NATURALIST 



[Vol. LV 



viduals. In the first place it is entirely uncertain from the 

 scanty accounts as t-o what time in development the quiescent 

 period occurs. Assuming that such a period does exist, it might 

 occur at some indifferent stage when no peculiar result would be 

 expected, for example after gastrulation, as it does in the bird 

 with no subsequent effect. In the light of the experimental 

 production of double individuals it is readily understood that 

 even though the egg of the deer is interrupted in its development 

 at an early stage, it might still be capable, on resuming develop- 

 ment, of giving a normal single embryo. A study of the experi- 

 mental production of twin and double individuals among fish 

 leads one to be surprised at the case of the armadillo, and to 

 ■expect the reaction found in the deer. The constant interrup- 

 tion 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 development without 

 noticeable modifications in structural response. A full considera- 

 tion of the different results following interruptions at critical 

 and indifferent developmental moments will be published in a 

 forthcoming number of the Amerkan Journal of Anatomy. 



In conclusion we may summarize the eases 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 the available oxygen supply. The inter- 

 ruption occurs at a critical period just preceding the primitive 

 streak and embryonic line formation. This egg appears to have 

 a decided tendency under conditions of arrest to form accessory 

 embryonic buds. As a result of the interaction of these external 

 and internal forces polyembryony is produced. 



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 



this egg. The cxlefiial I'a.-tors may be the same as in the case 

 of the ;irnia(lillu. hut tlu'v ititoract with differiMit internal factors 



