CHAPTER IV 



THEORIES OF POLYEMBRYONIC DEVELOPMENT 



IN DASYPUS 



A clue to a physiological explanation of polyembry- 

 ony appears to be presented by a comparison of the 

 developmental rates of uniparous and of polyembryonic 

 armadillos. In both species of Dasypus the gestation 

 period is abnormally long for a mammal of such com- 

 paratively small size, covering a period of from four to 

 five months; in Euphradus villostis, in which one egg 

 gives rise to but one embryo, the period of gestation is 

 only two months. So short is the gestation period that 

 two broods are produced in a season instead of one, as 

 in Dasypus. Fernandez attempts to explain the slower 

 development of Dasypus by saying that the nutriment 

 received by the embryos from the maternal blood is 

 not sufhcient to allow development to proceed at the 

 accustomed rate, and that the crowding of many 

 fetuses in a simple uterus tends further to retard develop- 

 ment. He forgets, however, that polyembryonic devel- 

 opment begins and is fully completed long before the 

 young egg establishes a permanent nutritive rehition 

 with the maternal tissues and that there is no crowding 

 in the uterus until the completely formed embryos have 

 attained a considerable degree of advancement. We 

 cannot, therefore, explain the establishment of four 

 or more growing points at the apex of the ectodcrmic 

 vesicle as due to insufficient maternal nutriment or to 

 crowding. 



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