EXPLANATION OF PLATE IO. XV11 



PLATE 9. 



Fig. i. Yolk of a Pristiurus egg with blastoderm and embryo. About two-thirds 

 of the yolk have been enveloped by the blastoderm. The embryo is still situated at 

 the edge of the blastoderm, but at the end of a bay in the outline of this. The thick- 

 ened edge of the blastoderm is indicated by a darker shading. Two arteries have 

 appeared. 



Fig. 2. Yolk of an older Pristiurus egg. The yolk has become all but enveloped 

 by the blastoderm, and the embryo ceases to lie at the edge of the blastoderm, owing 

 to the coalescence of the two sides of the bay which existed in the earlier stage. The 

 circulation is now largely developed. It consists of an external arterial ring, and an 

 internal venous ring, the latter having been developed in the thickened edge of the 

 blastoderm. Outside the arterial ring no vessels are developed. 



Fig. 3. The yolk has now become completely enveloped by the blastoderm. 

 The arterial ring has increased in size. The venous ring has vanished, owing to the 

 complete enclosure of the yolk by the blastoderm. The point where it existed is still 

 indicated (y) by the brush-like termination of the main venous trunk in a number of 

 small branches. 



Fig. 4. Diagrammatic projection of the vascular system of the yolk sac of a 

 somewhat older embryo. 



The arterial ring has grown much larger and the portion of the yolk where no 

 vessels exist is very small (x). The brush-like termination of the venous trunk is still 

 to be noticed. 



The two main trunks (arterial and venous) in reality are in close contact as in 

 fig. 5, and enter the somatic stalk close together. 



The letter a which points to the venous (blue) trunk should be v and not a. 



Fig- 5- Circulation of the yolk sac of a still older .embryo, in which the arterial 

 circle has ceased to exist, owing to the space outside it having become smaller and 

 smaller and finally vanished. 



EXPLANATION OF PLATE 10. (X. p. 298.) 

 COMPLETE LIST OF REFERENCE LETTERS. 



al. Alimentary canal, ch. Chorda dorsalis or notochord. ch'. Ridge of hypoblast, 

 which will become separated off as the notochord. ep. Epiblast. hy. Hypoblast. 

 //. Coalesced lateral and vertebral plate of mesoblast. m g. Medullary groove. 

 n. Nucleus of yolk, n a. Cells formed around the nuclei of the yolk to enter into the 

 ventral wall of the alimentary canal, n c. Neural or medullary canal, p v. Proto- 

 vertebra. so. Somatopleure. sp. Splanchnopleure. t s. Mesoblast of tail-swelling. 

 yk. Yolk-spherules. 



Figs, i a, i , ic. Three sections from the same embryo belonging to a stage 

 intermediate between B and C, of which fig. i a is the most anterior, (x 96 diameters.) 



The sections illustrate (i) The different characters of the medullary groove in the 

 different regions of the embryo. (2) The structure of the coalesced lateral and verte- 

 bral plates. (3) The mode of formation of the notochord as a thickening of the 

 hypoblast (ch'), which eventually becomes separated from the hypoblast as an 

 elliptical rod (i a, ch}. 



