FORMATION OF THE LAYERS. 285 



Figs. 8 a, Sl>, 8r. Three sections of the same embryo. Inserted mainly to illus- 

 trate the formation of the mesoblast as two independent lateral masses of cells ; only 

 half of each section is represented. 8 a is the most posterior of the three sections. 

 In it the mesoblast forms a large mass on each side, imperfectly separated from the 

 hypoblast. In 8 l>, from the anterior part of the embryo, the main mass oLmegpblast 

 is far smaller, and only forms a cap to the hypoblast at the highest point of the 

 medullary fold. In 8 c a cap of mesoblast is present, similar to that in 8 b, though 

 much smaller. The sections of these embryos were somewhat oblique, and it has 

 unfortunately happened that while in 8 a one side is represented, in 8^ and 8c the 

 other side is figured, had it not been for this the sections 8 b and 8 c would have been 

 considerably longer than 8 a. 



Fig. 9. Longitudinal section of an embryo belonging to a slightly later stage 

 than B. 



This section passes through one of the medullary folds. It illustrates the continuity 

 of the hypoblast with the remaining lower layer cells of the blastoderm. 



Figs, loa, lob, loc. Three sections of the same embryo belonging to a stage 

 slightly later than B, PL 8. The space between the mesoblast and the hypoblast 

 has been made considerably too great in the figures of the three sections. 



ic-tf. The most posterior of the three sections. It shews the posterior flatness 

 of the medullary groove and the two isolated vertebral plates. 



lob. This section is taken from the anterior part of the same embryo and 

 shews the deep medullary groove and the commencing formation of the ventral wall 

 of the alimentary canal from the nuclei of the yolk. 



\oc shews the disappearance of the medullary groove and the thinning out of 

 the mesoblast plates in the region of the head. 



Fig. ii. Small portion of the blastoderm and the subjacent yolk of an embryo at 

 the time of the first appearance of the medullary groove x 300. It shews two large 

 nuclei of the yolk () and the protoplasmic network in the yolk between them ; the 

 network is seen to be closer round the nuclei than in the intervening space. There 

 are no areas representing cells around the nuclei. 



Fig. ii. Nucleus of the yolk in connection with the protoplasmic network 

 hardened in osmic acid. 



Fig. 13. Portion of posterior end of a blastoderm of stage B, shewing the forma- 

 tion of cells around the nuclei of the yolk. 



Fig. 14. Section through part of a young Scyllium egg, about t V tn of an inch in 

 diameter. 



/. Protoplasmic network in yolk. zp. Zona pellucida. eh. Structureless 

 chorion. fcp. Follicular epithelium, x. Structureless membrane external to this. 



