No. 636] POLYEMBRYONY IN THE ARMADILLO 63 



nandez.2 The growth and expansion of these twin studies lias 

 brought our understanding of the phenomena of polyembryouy 

 in the armadillo to a considerable state of maturity. 



These authors readily agreed that in most species of aniiiidillo 

 the individual members of a litter, usually four in tlu' Trxas 

 species and eight in the common South Amei-ienn fonu. ;ir.^ all 

 derived from a single egg. It required considera.blc otVoi-t. how- 

 ever, to obtain the material that would furnish tlie morphologi- 

 cal st-ages of the process by which the polyembryonic development 

 was accomplished. We are finally indebted to Patterson,^ for 

 the very thorough and satisfactory manner in which he has col- 

 lected and studied the early embryonic conditions ; and particu- 

 larly for having shown the first stages of the budding process 

 through which the single blastocyst gives rise to four distinct 

 embryonic areas, each exhibiting a typical primitive streak 



In connection with the fish experiment it now becomes im- 

 portant to ascertain exactly what degree of development has 

 been attained by the armadillo blastocyst at the time the bud- 

 ding process begins. And since, according to ray interpretation, 

 these buds should arise at the time of gastrulation or blastopore 

 formation, it becomes necessary to consider very briefly the 

 germ-layers and gastrulation in mammals. The decidedly pre- 

 cocious and highly modified method of forming the primary 

 germ-layers in the mammalian blastocyst is not strictly com- 

 parable to gastrulation or the method of germ-layer formation 

 found among the other vertebrates. On the other hand, the em- 

 bryonic line or primitive streak of the mammalian egg is ex- 

 actly comparable to the blastopore and head proeess formation 

 in the simpler forms. 



The blastocyst of the armadillo has already, by a process of 

 cell migration and delamination, separated off the primary ento- 

 derm from the ectoderm and further modified these layers be- 

 fore the budding which forms the embryonic primordia has 

 begun. The primordia are first formed by a thickening of the 

 ectodermal layer of the bla.stocyst. The primarv entoderm then 

 invaginates into the primordia to form the secondarv entoderm 

 of the gut. The precocious cell migration and splitting into 

 layers in the mammal's egg is associated with the early implanta- 



2 M. Fernandez, Morph. Jahrh., Bd. 39, p. 302, 1909. 



3 J. T. Patterson, Jour. Morph., Vol. 24, p. 559, 1913. 



