64 



THE AMERICAN NATUEALIST 



[Vol. LV 



tion of the embryo upon the uterme-wall of the mother, and the 

 later primitive streak formation may be interpreted as related 

 to the actual gastrulation or blastopore formation away from 

 which the line of the embryo always develops. 



Whether the validity of the above briefly outlined interpreta- 

 tion of the germ-layer formation is admitted or not, we have 

 in the armadillo a process of budding taking place from the 

 blastoderm and associated with accessory or extra blastopore 

 formation in much the same way as are the accessory embryos 

 along the germ-ring in the egg of the bony-fish. These buds 

 also accord with Kopsch's description of a double gastrular 

 condition with two blastopores in a blastoderm of Lacerta agilis, 

 from which he concluded that twin formation as well as anterior 

 duplication arises from a double Einstiilpungen. And further, 

 Assheton has described a similar condition in a blastodermic 

 vesicle of the sheep. He, however, imagined the condition to 

 have been due to a splitting during the morula stage. 



The double primitive streaks in the hen's egg and other 

 forms all lend themselves to strengthen the interpretation that 

 double embrj^o formation first asserts itself by a double gastrula- 

 tion or blastopore formation, which is initially a process of 

 double instead of single bud formation. Patterson's description 

 of the origin of the quadruplet buds in the Texas armadillo 

 furnishes the most striking case in the study of these conditions. 

 And we may conclude that the budding or accessory embryo 

 formation in the egg of the armadillo is exactly the same develop- 

 mental process as that which gives rise to twins and double indi- 

 viduals in other vertebrate eggs. 



However, the very important question yet remains to be an- 

 swered. Why does this accessory bud formation occur so con- 

 stantly in the Texas armadillo in contrast to the single embryo 

 formation of mammalian eggs in general? Patterson failed to 

 answer this question, but he supplied some very significant data 

 which Newman,* has appreciated as being intimately connected 

 with the occurrence of polyembryony. 



In connection with the collection of material Patterson^ dis- 

 covered a "period of quiescence" of the omhryonic blastocyst. 



