Development of the Nine-Banded Armadillo. 371 



IV. THE EARLY EMBRYOLOGY 



In the development of the nine-banded armadillo we find that 

 striking peculiarity, met with in the rodents, of germ-layer inver- 

 sion. In the case of the armadillo the inversion is intimately 

 bound up with the formation of the four embryos, and without 

 it the mechanics of specific polyembryony, as found here, would 

 be inexplicable. The possession of a common amnion by the em- 

 bryos at an early stage could only occur as a sequence to inversion, 

 and strongly suggests that the embryos are the product of a sin- 

 gle fertilized egg. 



In the present' description of Tatu novemcinctum we shall 

 begin with the 'primitive streak stage, and leave out of account 

 the younger embryos (except for a brief reference to the work of 

 Fernandez) until we shall have secured a series covering that im- 

 portant period. In dealing with the following stages considerable 

 emphasis is placed upon the embryological details, and especially 

 upon the relations existing between the embryos. This is done 

 because these stages furnish the strongest internal evidence for 

 polyembryony that has been brought forward. 



A. The Earliest Stages of Fernandez 



It will be necessary to refer to the work of Fernandez, especially 

 to the part in which he describes his youngest two stages; because 

 they hold the key not only to the morphology of the older embryos 

 of Tatu hybridum, but also, we believe, to that of the stage of 

 T. novemcinctum which we are about to consider. 



Fernandez secured two specimens of his earliest stage, and the 

 one he describes in detail was cut longitudinally into twenty- 

 three sections (10 microns thick). It was found attached to the 

 mucus membrane at the bottom of a fold at* the fundus end of 

 the uterus. 



Fernandez correctly interprets the condition presented in this 

 early stage as one having been brought about through the process 

 of germ-layer inversion, and compares the vesicle to corre- 



