io6 THE PHYSIOLOGY OF TWINNING 



were non-sensitive no placentation would result. In 

 this case, I believe, the non-sensitiveness is not of the 

 embryo but of the maternal mucosa. It is slow to 

 acquire the sensitivity that a uterus normally possesses 

 in the presence of a blastocyst. Recent experimental 

 work has tended to show that the sequence of events in 

 mammalian gestation is controlled by an intricate 

 system of hormones. Th^ event of ovulation is quickly 

 followed by the formation of a glandular body, the 

 corpus luteum, which appears to control by its secretions 

 further ovulation and to excite the uterine mucosa to 

 co-operate with the blastocyst in placentation. If, for 

 any reason, the functioning period of the corpus luteum 

 should be delayed there would inevitably follow a delay 

 in placentation. In my opinion this is a point of experi- 

 mental attack. If my assumption be correct, early 

 removal of the corpus luteum might be expected to 

 inhibit placentation. In the armadillo we probably 

 have a case of sluggish activity of the corpus luteum; 

 for it grows very large and becomes very active at a 

 later period, and excellent placentation finally results. 

 Even if it should prove to be true that slow growth and 

 sluggish activity of the corpus luteum is a part of our 

 causal chain we would still have to discover the cause 

 of this. The search for causes is endless. 



The essentials in the causal chain of twinning in the 

 armadillo, according to the foregoing theory, are as 

 follows: (a) slow development or sluggish functioning 

 of the corpus luteum; (b) failure of the maternal mucosa 

 to respond to the presence of the blastocyst; (c) belated 

 placentation; (d) cessation of development for about 

 three weeks; (e) partial loss of polarity or deaxiation 



