EXPLANATION OF PLATES 6 AND 7. 



EXPLANATION OF PLATE 6. (X. p. 222.) 



Fig. I. Section through the germinal disc of a ripe ovarian ovum of the Skate. 

 gv. germinal vesicle. 



Fig. 2. Surface-view of a germinal disc with two furrows. 



Figs. 3, 4, J. Surface-views of three germinal discs in different stages of segment- 

 ation. 



Fig. 6. Section through the germinal disc represented in fig 3. n. nucleus ; x. edge 

 of germinal disc. The engraver has not accurately copied my original drawings in 

 respect to the structure of the segmentation furrows. 



Figs. 6a and 6/'. Two furrows of the same germinal disc more highly magnified. 



Fig. 6r. A nucleus from the same germinal disc highly magnified. 



Fig. 7. Section through a germinal disc of the same age as that represented in 

 fig. 4. ?!. nucleus; «jr. modified nucleus ; hj'. modified nucleus of the yolk; yl furrow 

 appearing in the yolk around the germinal disc. 



Figs. 7 a, 7^, 7 c Three segments with modified nuclei from the same germin.al 

 disc. 



Fig. 8. Section through a somewhat older germinal disc. cp. epiblast ; n' . nuclei 

 of yolk. 



Figs. 8 rt, Si, ?>c. Modified nuclei from the yolk from the same genninal disc. 



Fig. 8 d. Segment in the act of division from the same germinal disc. 



Fig. 9. Section through a germinal disc in which the segmentation is completed. 

 It shews the larger collection of cells at the embryonic end of the germinal disc than 

 at the non-embryonic, ep. epiblast. 



EXPLANATION OF PLATE 7. (X. p. 246.) 



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. Hypolilast. //. Lower layer cells, ly. Line 

 separating the yolk from the blastoderm, m. Mesoblast. mg. Medullary groove. 

 «'. 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. bJ. Large cell resting on the 

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

 diameters. (Hardened in chromic acid.) 



