380 H. H. Newman and J. T. Patterson. 



been said above concerning the transverse sections, this may be 

 studied with profit. The entoderm in this section can be traced 

 from the protochordal plate back along the entire length of the 

 embryo. Throughout the greater part of its length it is composed 

 of flattened cells, but near the posterior end of the primitive streak 

 these cells become cuboidal, and in the region of the mouth of 

 the allantoic tube (al) take on a columnar appearance. Posterior 

 to the allantoic opening the yolk-sac passes back and ends abrupt- 

 ly at the margin of the Trager epithelium (tr. e.}. 



WMle the median section does not show the lateral belly- 

 stalk bands which form the main connections between the embryo 

 and the Trager, it does, however, bring out with clearness the 

 union between these two as seen at the extreme tip of the embryo. 

 This connection (ms. co.) is simply a backward and downward 

 continuation of the allantoic mesoderm, which passes over into 

 the general mesodermal lining of the Trager region. 



C. The Five to Seven Somite Stage 



The general relations existing between the various parts of the 

 embryonic vesicle in this stage closely resemble those of the pri- 

 mitive streak stage, but the vesicle is almost twice as large, 

 measuring 15 mm. long by 14 mm. wide (fig. 14). Owing to this 

 increase the horns are not only relatively but actually shorter 

 than in the preceding stage. The Trager has undergone marked 

 differentiation and shows a tendency to overgrow the yolk-sac 

 region. The common amnion with its canals presents the same 

 general features as before. 



The most interesting changes have occurred in connection with 

 the development of the embryos, and it is to these that we would 

 direct attention. In the first place emphasis should be placed upon 

 the fact that the embryos are not equally differentiated, for the 

 dorsal and left lateral have each, five pairs of primitive segments 

 while the ventral and right lateral embryos have seven. In other 

 words, the individuals of the same pair are in the same stage 

 of development. 



