THE EARLY EMBRYOLOGY OF THE MOUSE 13 



ment, actually there is no very fundamental change in the relations of the 

 important structures. Those changes that are involved are easily under- 

 stood from a comparison of the early development of the mouse with that of a 

 primitive rodent, the thirteen-striped ground squirrel. Three comparable 

 stages for each species are shown diagrammatically in Fig. 9. Beginning 

 students of embryology will want to refer again to this figure after complet- 

 ing the study of later stages in the mouse. > 



In primitive rodents, as represented by the thirteen-striped ground 

 squirrel, the embryonic area (embryonic ectoderm and underlying entoderm) 

 forms a disc that overlies an almost spherical yolk cavity . In the mouse , the 

 embryonic area forms a deep cup pushed far down into the yolk cavity, 

 which thereby is greatly reduced in size. The obvious explanation of this 

 condition is that during some period in the evolution of the mouse there 

 developed an invagination of the embryonic area into the yolk cavity, the 

 curvature of the embryonic area thereby being reversed and the relation of 

 ectoderm and entoderm inverted. The change is comparable to that 

 produced when a rubber ball has one side pushed in, being altered thereby 

 from a sphere to a cup. 



In Fig. 9D the lumen of the egg cylinder is shown extending through the 

 ectoplacental cone to the outside. This condition is probably the exception 

 rather than the rule, but it has been described by Sobotta (61) and Melissinos 

 (41), and we have found it in a few cases in our material. It is significant 

 evidence for the theory that the inversion of the germ layers is due to an 

 invagination of the embryonic area. 



Further evidence is provided by the later development of the thirteen- 

 striped ground squirrel (Fig. 9C). In this species the whole embryo sinks 

 down into the yolk cavity, carrying the splanchnopleure with it. The 

 splanchnopleure is thereby inverted, but no inversion of embryonic ectoderm 

 and entoderm occurs because of the advanced development of the embryo at 

 the time. However, if the sinking or invagination of the embryonic area 

 were pushed back to an earlier period of development, the condition found 

 in the mouse would result. 



One interesting consequence of the inversion of the germ layers is the 

 production of a very compact form of early development. Much seemingly 

 waste space in the yolk cavity is eliminated. The reader should note in this 

 connection that the drawings of the mouse embryos in Fig. 9 are at a higher 

 scale of magnification than those of the ground squirrel embr}'os. Actually, 

 at comparable stages of early development, the total volume of a mouse 

 embryo is, in round figures, perhaps one-fiftieth that of the total volume of 



