DESCRIPTION OF PLATES VI— IX— continued. 



Pigs. 20, 21, 22, and 23.^Are sections through the centre of the inner 

 mass of four vesicles. They show the formation of the hypoblast from inner 

 mass cells. In the two latter the outer layer cells above the inner mass have 

 increased in number and size, and the cells of the latter are becoming 

 columnar. 



In fig. 23 the secondary cavity is shown. 



Figs. 22 and 23 are Miiller's fluid preparations. 



Fig. 2i. — Section through the centre of the inner mass drawn in fig. 5. 

 The secondary cavity contains cells, which are homologous with the cells of 

 the trager of inverted types. The inuer mass, in the form of an hemispherical 

 plate, is continuous at the edges with the outer layer, and is composed of a 

 double row of columnar cells. The hypoblast is now a distinct membrane, 

 extending laterally beyond the inner mass. The zona is much thinner. 



Fig. 25. — Section through the area drawn in fig. 7. The arched plate is 

 becoming extended and flattened. In this specimen no cells are shown in the 

 secondary cavity, but the outer layer above the cavity is much thickened. 



Fig. 26. — Section through an inner mass still more flattened. Ceils are 

 present in the secondary cavity, and show signs of becoming incorporated 

 with the inner mass. The cells of the outer layer at the edges give similar 

 indications {(. c). 



Fig. 27. — Section a little to one side of the middle line through an inner 

 mass, in which the secondary cavity is almost entirely obliterated. The inner 

 mass is continuous at the edges with the outer layer, and several cells {i. c.) 

 may be interpreted as outer layer cells, becoming incorporated vnth the inner 

 mass. Compare fig. 49. 



Fig. 28. — Transverse section through the embryonic area of the vesicle 

 drawn in fig. 8. The cells of the inner mass have now united with the outer 

 layer cells above them to form the epiblast plate of the embryonic area. The 

 plate is not now uniformly two layers thick. The zona can no longer be dis- 

 tinguished. At the side one of the papillae seen in fig. 8 is shown in 

 section. 



Fig. 29.— Section of another papilla of the vesicle drawn in fig. 8. 



Fig. 30. — Transverse section through the embryonic area drawn in surface 

 view in fig. 10. It is throughout composed of epiblast and hypoblast. 



PLATE VIII. 



The figures on this plate are all magnified 265 diameters, with the excep- 

 tion of fig. 32, which is magnified 170 times. 



Fig. 31.— Longitudinal section through the middle line of part of an em- 

 bryonic area in which the primitive streak has begun to form. In front of the 



