PLATE LXXIV. 



FIG. 174. Section of the cover or dome of Rusconi's cavity (Bufo). a. Corneal layer. 6. Nervous 

 layer, c. Motor-germiuative layer, d. Epithelial glandular layer, c and d are the offspring of formative 

 elements. 



FIG. 175. Vertical section of a portion of the area pellucida and area opaca of the blastoderm of a fresh- 

 laid hen's egg. In the section corresponding to the area pellucida, the blastoderm consists of two distinct 

 layers, a the upper, and 6 the lower; the latter looser and consisting of larger elements, cc. Formativ 

 elements lying on the floor of the cleavage cavity F, which have originated from the geim, and are filled with 

 yolk granules. These elements are continuous with similar ones in the area opaca. 



FIU. 176. Section of blastoderm of lien's eg<c, at the fifteenth hour of incubation, a. Tinker, and !> Lower 

 ayer. c. Cleavage cavity. </. Yolk rim. /. Formative elements on the floor of the cleavage cavity, g. Similar 

 elements which have already migrated in l>etweeu the layers of the blastoderm. 



FIG. 177. Section of commencing embryo at the twenty-sixth hour after incubation, a. I'pper. l> middle, 

 c under layer, d. Central portion of the middle layer, whirh is here fused with the upper layer, e. Primitive 

 groove. /. Dorsal ridges. 



