46 EDWARD MCCRADY, JR. 



eutherian vesicle, and was possible, I believe, only in the 

 absence of a complete series of sectioned specimens. In a 

 study of external views it is easy to be deceived on this point, 

 and, for my" own part, I accepted the old interpretation with- 

 out misgiving until I had studied sections of stage 22 (speci- 

 men 16139) which made the error obvious. 



In the opossum there is no differentiation of embryonic and 

 trophoblastic cells. The entire vesicle corresponds to the 

 ovum of a fish, an amphibian, a reptile, or a bird, and no line 

 can ever be drawn between the cells which are to be retained 

 in the definitive body and those which are to be discarded. 



As mentioned in the first part of this chapter the unilaminar 

 blastocyst shows no polarity or visible differentiation of any 

 sort. Probably most of the cells at this time are indifferent 

 protodermal cells just as Mangold ('24) has shown most of 

 the cells of the amphibian egg are at the beginning of gastru- 

 lation. But a few have a peculiarity 7 which makes them 

 later enlarge and migrate to the interior. These consti- 

 tute the endoderm and probably also part of the axial meso- 

 derm, the prechordal plate, which though embedded in the 

 endoderm may be seen proliferating mesoderm in stage 22. 

 The 'invaginated' material then induces the formation of a 

 medullary plate in the hitherto pluripotential external 

 material. 



These facts are perfectly parallel to what is known to occur 

 in the lower vertebrates, and they make the primitive mam- 

 malian blastocyst fit naturally into the phylogenetic series. 



T I do not mean, of course to assert that the peculiarity which causes the endo- 

 dermal mother-cells to enlarge and migrate into the blastocoel will never be traced 

 back to some sort of cytoplasmie localization in the uncleaved ovum. Nor do I 

 mean that there is any a priori objection to the idea that such material, if 

 existent, may be consistently allotted to one of the first to blastomeres. All I mean 

 is that this peculiarity which causes some cells to migrate internally and subse- 

 quently to induce a medullary plate would correspond to the primary organizer, 

 not to the whole embryo, and that we do not yet know whether the plane of 

 the first cleavage bears any constant relation to any such material or not. In 

 the only mammal for which experimental evidence is at present available 

 (the rat), Nicholas ('34) has shown that either of the first two blastomeres can 

 produce a whole embryo. 



