258 CHARLES R. STOCKARD 



5. The bird's egg is usually laid, according to investigations 

 on this subject, after the process of gastrulation has commenced. 

 Yet double chick embryos are not uncommon among the develop- 

 mental stages observed in the laboratory, although in nature such 

 specimens almost never exist at the time of hatching. 



The cause for the double chick embiyos is the same, I believe, 

 as that indicated above in the case of the double fish. Although 

 the great majority of hen's eggs are laid and their development 

 stopped by the fall in temperature after gastrulation has begun, 

 still it is recognized by those who have investigated the subject 

 that there is considerable variation in the developmental stages 

 of the eggs at the time of laying, and a minority of eggs are laid 

 before gastrulation has begim. When an egg in this stage is 

 stopped by the fall in temperature following laying, it would be 

 expected from the experience with the fish that just such eggs 

 would frequently give rise to two points of gastrulation and two 

 embryonic fundaments instead of one. The interruption in the 

 process of development at this critical time and the resumption 

 of development at an equally slow rate in all regions of the blasto- 

 derm, permits more than one potential embryo-forming region 

 to express itself. The interruption at this particular moment is 

 the very probable cause of twin and double specimens. 



6. Polyembryony in the armadillo is in all probability explain- 

 able on a similar basis to the cases above. Development begins 

 as in most other mammals in the fallopian tubes and continues 

 until the egg passes down into the uterus as an early blastocyst. 

 Development then stops in the armadillo for a period of several 

 weeks with the blastocyst lying free in the uterus, as Patterson 

 ('13) has reported. The stop here is not due to a temperature 

 change, since none has occurred, but is very probably on account 

 of an exhaustion of the original oxygen supply derived from the 

 ovarian blood. The uterus fails to react immediately to the pres- 

 ence of the blastocyst, implantation is delayed, and no means of 

 obtaining oxygen necessary for continuing development is pos- 

 sible until the egg becomes implanted. After the delayed implan- 

 tation has taken place, development is slowly resumed in a way 

 which gives rise to multiple embryo formations or budding, as 



