376 H. H. Newman and J. T. Patterson. 



an enormous extension, due in part to the natural growth of the 

 vesicle and in part to the modification in the shape of the Trager 

 wall, which has changed from concave to convex; on the other 

 hand, the common amnion not only has failed to keep pace with 

 this rapid expansion of the embryonic vesicle, but has actually 

 ceased to grow at all, and is destined soon to degenerate and dis- 

 appear. In the rapid growth of the embryonic vesicle the embryos 

 gradually have been drawn away from the common amnion, 

 and consequently their connections with it have been pulled out 

 into the long, slender, tube-like canals. 



The embryo viewed from the dorsal side shows the exact rela- 

 tions existing between it and the amnion (fig. 13)-. In general 

 outline the embryo is slipper-shaped and throughout the greater 

 part of its length the amnion conforms to this contour. Both an- 

 teriorly and posteriorly the amnion narrows down rapidly in 

 the former direction to produce the amniotic canal (am. c. c.) 

 and in the latter to form the posterior amniotic process (p. am. .p), 

 which ends blindly above the Trager. The level at which the am- 

 nion becomes narrower than the belly-stalk varies in different 

 embryos. In the embryo in question it cuts in some distance 

 posterior to the mouth of the allantois, but in other cases it may 

 cut in at a level somewhat anterior to this point. 



The entire embryo, fr^m the anterior end of the medullary plate 

 to the posterior tip of the amnion, measures 3.5 mm., but the 

 embryo proper is only 2.5 mm. long. Running through the cen- 

 tral part of the medullary plate is the elongated primitive streak, 

 in which is a well developed primitive groove with a faintly de- 

 fined primitive pit at its anterior end. The primitive streak is 

 exactly 1 mm. long, and has at its anterior end a distinct head 

 process measuring 0.28 mm. 



The outline of the allantois is seen through the embryo, and 

 begins a short distance back of the posterior end of the primitive 

 streak and extends through the mesoderm of the belly-stalk, fi- 

 nally ending some distance anterior to the tip of the amnion. Fer- 

 nandez does not describe the development of the allantois in the 

 Mulita, and this stage is, of course, too far advanced to give any 

 clue to the exact nature of its origin. 



