STRTJCTURE AND DEVELOPMENTAL RATE 191 



decidedly precocious and highly modified method of forming the 

 primary germ-layers in the mammalian blastocyst is not strictly 

 comparable to gastrulation or the method of germ-layer forma- 

 tion found among the other vertebrates. On the other hand, 

 the embryonic line or primitive streak of the mammalian egg is 

 exactly comparable to the blastopore and embryonic process 

 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 before 

 the budding which forms the embryonic primordia has begun. 

 But it is in the primordia that the invagination of the entoderm 

 forms the secondary entoderm of the gut and the embryonic 

 mesoderm arises from a typical primitive-streak region much as 

 in lower vertebrates. The precocious cell migration and split- 

 ting into layers in the mammal's egg is associated with the early 

 implantation of the embryo upon the uterine 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 blasto- 

 derm and associated with accessoiy 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 ('95), 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 duplica- 

 tion arises from a double gastrula — 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 sphtting 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 

 embryo formation first asserts itself by a double gastrulation 

 or blastopore formation, which is initially a process of double 



