4O DEVELOPMENT AND GROWTH OF THE BLASTODERM. 



EXPLANATION OF PLATE I. Figs. 15 and 912. 



Fig. i. Section through an unincubated blastoderm, shewing the upper layer, 

 composed of a single row of columnar cells, and the lower layer, composed of several 

 rows of rounded cells in which no nucleus is visible. Some of the " formative cells," 

 at the bottom of the segmentation cavity, are seen at (6). 



Fig. 2. Section through the periphery of an eight hours' blastoderm, shewing the 

 epiblast (/), the hypoblast (h), and the mesoblast commencing to be formed (c), partly 

 by lower-layer cells enclosed between the epiblast and hypoblast, and partly by 

 formative cells. Formative cells at the bottom of the segmentation cavity are seen 

 at b. At s is one of the side folds parallel to the primitive groove. 



Fig. 3. Portion of the hypoblast of a thirteen hours' blastoderm, treated with 

 silver nitrate, shewing the great variation in the size of the cells at this period. An 

 hour-glass shaped nucleus is seen at a. 



Fig. 4. Periphery of a twenty-three hours' blastoderm, shewing cell for cell the 

 junction between the hypoblast (//) and white-yolk spheres (w). 



Fig- 5- Junction between the white-yolk spheres and the hypoblast cells at the 

 passage from the area pellucida to the area opaca. The specimen was treated with 

 silver nitrate to bring out the shape of the cells. The line of junction between the 

 opaque and pellucid areas passes diagonally. 



Fig. 9. Section through the primitive streak of an eight hours' blastoderm. The 

 specimen shews the mesoblast very much thickened in the immediate neighbourhood 

 of the primitive streak, but hardly formed at all on each side of the streak. It also 

 shews the primitive groove just beginning to be formed (pr), and the fusion between 

 the epiblast and the mesoblast under the primitive groove. The hypoblast is com- 

 pletely formed in the central part of the blastoderm. At f is seen one of the side 

 folds parallel to the primitive groove. Its depth has been increased by the action of 

 the chromic acid. 



Fig. 10. Hypoblast cells from the hinder end of a thirty-six hours' embryo, treated 

 with silver nitrate, shewing the regularity and elongated shape of the cells over the 

 embryo and the smaller cells on each side. 



Fig. n. Epiblast cells from an unincubated blastoderm, treated with silver 

 nitrate, shewing the regular hexagonal shape of the cells and the small spherules 

 they contain. 



Fig. 12. Portion of the epiblast of a thirty-six hours' embryo, treated with silver 

 nitrate, shewing the small rounded cells frequently found at the meeting-points of 

 several larger cells which are characteristic of the upper layer. 



