PLATE 5 



EXPLANATION OF FIGURES 



55 Median section of specimen No. 289. The two deeply-staining cells lying 

 at about the center of the extraembryonic cavity are degenerating entodermal cells 

 the single cell situated toward the right side of this cavity is of ectodermal origin, 

 and indicates the beginning of a cell proliferation which will give rise to the extra- 

 embryonic mesoderm. X 106. 



56 Median section from specimen No. 298. It shows an early stage in the 

 development of the mesoderm, which appears in the form of small vesicles. X 85. 



57 A section lying four sections to the left of the preceding, and showing well- 

 formed but small mesoderm vesicles. X 106. 



58 Median section, taken in a right-left plane, of No. 234. This shows two rel- 

 atively large mesodermal vesicles, which have been formed by a fusion of smaller 

 vesicles, such as appear in the preceding figure. X 106. 



070 



