38 THE BIOLOGY OF TWINS 



In general, we may say in concluding this account 

 of the history of the egg up to the beginning of cleavage 

 that there is nothing in any way suggestive of poly- 

 embryony about any phase of the process. The eggs 

 are ovulated singly, have small polar bodies, are ferti- 

 lized by but one sperm, and begin cleavage in normal 

 fashion, if we may judge by the data derived from a 

 study of parthenogenetic cleavage. 



EARLY EMBRYONIC DEVELOPMENT 



Fernandez^ was the first to describe for any species 

 of armadillo embryos in which no beginnings of poly- 

 embryonic fission had taken place. He studied several 

 early embryos of Dasypus hybridus, and although his 

 material was in rather poor condition, it was clear that 

 the blastodermic vesicle was a single individual in the 

 stage examined and not unlike similar stages in some 

 of the rodents. In the autumn of 1909 Newman 

 and Patterson^ obtained a number of stages of Dasypus 

 novemcinctus covering the period from the primitive 

 streak on to birth. In the following season a number 

 of earlier stages were collected, some of the youngest 

 of which were lost in fixation. At the end of this 

 season these two collaborators divided forces on account 

 of the removal of the senior author from Texas to 

 Chicago. In the resultant division of the work the 

 completion of the study of embryonic development 

 was assigned to Patterson and the genetic problems 

 to Newman. It took Patterson two more seasons to 



^ Fernandez, loc. cit. 



2 H. H. Newman and J. T. Patterson, Journal of Morphology, 

 XXI (1910). 



