374 H. H. Newman and J. T. Patterson. 



must be attributed to the incompleteness of the history of these 

 early stages a fact which Fernandez freely admits. 



Within the limits of the vesicle there are two distinct cavities : 

 one the general cavity of the vesicle (ex. c.), and the other the com- 

 mon amniotic cavity (c. am. c.). The former is lined throughout 

 with mesoderm, and the latter with ectoderm. 



The embryos, which are in the medullary plate stage, lie in 

 pocket-like diverticula from the lateral margins of the floor of 

 the common amnion; and each embryo is connected with the lat- 

 ter by a short tube, which is the beginning of the amniotic con- 

 necting canal. The common amnion, together with its accom- 

 panying embryos, is the product of the inner ectodermal sac of 

 the earlier stage. It is not at all easy to explain fully the manner 

 in which the various structures presented in this vesicle develop 

 out of the primordia of the preceding vesicle, although the history 

 of several of them is self evident. To go from this to the succeed- 

 ing stage is, however, an easy step, and we shall therefore pass 

 directly to it as exemplified in our youngest vesicle of Tatu 

 novemeinctum. 



B. The Primitive Streak Stage 



We were fortunate in being able to secure from the uterus the 

 entire embryonic vesicle in practically a perfect state of preserva- 

 tion. The opportunity was thus afforded not only to make a de- 

 tailed study of the relations existing between the different embryos 

 but also to obtain a drawing of the vesicle as a semi transparent 

 object (fig. 12). In the preserved condition it measured 7 mm. 

 wide by 9 mm. long. It is slightly flattened dorso-ventrally but 

 in general outline is shaped like an inverted balloon, with two lat- 

 eral horn-like projections which fit into the openings of the fal- 

 lopian tubes. These horns persist for a considerable time and are 

 of great service in aiding one to maintain the correct orientation 

 of the vesicle during its early development. 



The surface of the vesicle presents two distinct regions, the 

 lower of which fits into the fundus end of the uterus and is recog- 

 nized as the Trager. It is therefore covered by Trager epithe- 



