CAUSES OF TWINNING IN ARMADILLOS 105 



dently the egg goes as far on its own resources as it can, 

 and would, in any species of mammal, cease to develop 

 if unable to get the necessary assistance furnished by 

 placentation. What the nature of early placental aids 

 to development are can readily be conjectured. They 

 are primarily food and oxygen and a means of eliminating 

 wastes. Doubtless, in the armadillo, the deprivation 

 of all of these growth necessities brings growth to a nearly 

 complete standstill. Patterson notes that cell division 

 ceases but that the vesicle continues to expand through 

 the excretion of liquid within the cavity of the vesicle. 

 Protoplasmic activity must be going on all this time, 

 but cell respiration must be largely anaerobic. Probably 

 there is an accumulation of carbon dioxide and other 

 metabolic wastes. This of itself would tend to check 

 development. Stockard claims that the primary cause 

 of the developmental arrest is lack of oxygen, and this 

 may well be partially true. The real cause, however, 

 is failure to attain at the proper time the essential 

 growth stimulus which is normally supplied by placenta- 

 tion. 



When we have added as a link to our chain of 

 causes that of belated placentation, we still must account 

 for this situation. Why does not the egg attain a 

 placental connection when it first comes in contact with 

 the maternal mucosa ? This is a problem whose solution 

 would get at the very foundations of the causes of twin- 

 ning and might lead to an experimental control of 

 twinning in mammals, including man. When blasto- 

 cysts undergo placentation there is a mutual proHf eration 

 of tissues, both embryonic and maternal. Each seems 

 to respond to stimuli given off by the other. If either 



