284 DEVELOPMENT OF ELASMOBRANCH FISHES. 



EXPLANATION OF PLATE 7. 

 COMPLETE LIST OF REFERENCE LETTERS. 



c. Cells formed in the yolk around the nuclei of the yolk. ep. Epiblast. er. Em- 

 bryonic ring. es. Embryo swelling, hy. Hypoblast. //. Lower layer cells, ly. Line 

 separating the yolk from the blastoderm, in. Mesoblast. nig. Medullary groove. 

 n'. Nuclei of yolk. na. Cells to form ventral wall of alimentary canal which have 

 been derived from the yolk, n al. Cells formed around the nuclei of the yolk which 

 have entered the hypoblast. sc. Segmentation cavity, vp. Combined lateral and 

 vertebral plate of mesoblast. 



Fig. i. Longitudinal section of a blastoderm at the first appearance of the seg- 

 mentation cavity. 



Fig. i. Longitudinal section through a blastoderm after the layer of cells has 

 disappeared from the floor of the segmentation cavity, bd. Large cell resting on the 

 yolk, probably remaining over from the later periods of segmentation. Magnified 60 

 diameters. (Hardened in chromic acid.) 



The section is intended to illustrate the fact that the nuclei form a layer in the yolk 

 under the floor of the segmentation cavity. The roof of the segmentation cavity is 

 broken. 



Fig. ia. Portion of same blastoderm highly magnified, to shew the characters of 

 the nuclei of the yolk n' and the nuclei in the cells of the blastoderm. 



Fig. i b. Large knobbed nucleus from the same blastoderm, very highly magnified. 



Fig. 2 c. Nucleus of yolk from the same blastoderm. 



Fig. 3. Longitudinal section of blastoderm of same stage as fig. 2. (Hardened in 

 chromic acid.) 



Fig. 4. Longitudinal section of blastoderm slightly older than fig. 2. Magnified 

 45 diameters. (Hardened in osmic acid.) 



It illustrates (i) the characters of the epiblast ; (2) the embryonic swelling; (3) 

 the segmentation cavity. 



Fig. 5. Longitudinal section through a blastoderm at the time of the first appear- 

 ance of the embryonic rim, and before the formation of the medullary groove. 

 Magnified 45 diameters. 



Fig. 5 a. Section through the periphery of the embryonic rim of the blastoderm 

 of which fig. 5 represents a section. 



Fig. 6. Section through the embryonic rim of a blastoderm somewhat younger 

 than that represented on PI. 8, fig. E. 



Fig. 7. Section through the most projecting portion of the embryonic rim of a 

 blastoderm of the same age as that represented on PI. 8, fig. B. The section is drawn 

 on a very considerably smaller scale than that on fig. 5. It is intended to illustrate 

 the growth of the embryonic rim and the disappearance of the segmentation cavity. 



Fig. 7 a. Section through peripheral portion of the embryonic rim of the same 

 blastoderm, highly magnified. It specially illustrates the formation of a cell (c) 

 around a nucleus in the yolk. The nuclei of the blastoderm have been inaccurately 

 rendered by the artist. 



