EXPLANATIOX OF PLATE V. 



ep. epiblast. 



m. mesoblast. 



hy. hypoblast. 



vp. combined lateral and vertebral plate of mesoblast. 



na. cells to form ventral wall of alimentary canal which have been 

 derived from the yolk. 



ly. line separating the yolk from the blastoderm. 



mg. medullary groove. 



71. nucleus of yolk. 



c. cells formed in the yolk around the nuclei of the yolk. 



10 «, 10 &, 10 c. Three sections of the same embryo belonging to a stage 

 slightly later than B, PI. vi. The space between the mesoblast and the 

 hypoblast has been made considerably too great in the figures of the three 

 sections. 



10 «. The most posterior of the three sections. It shows the posterior 

 flatness of the medullary groove and the two isolated vertebral plates. 



10 &, This section is taken from the anterior part of the same cinbrvo 

 and shows the deep medullary groove and the commencing formation of 

 the ventral wall of the alimentary canal from the nuclei of the yolk. 



10 c shows the disappearance of the medullary groove and the 

 thinning out of the mesoblast plates in the region of the head. 



11. Small portion of the blastoderm and the subjacent volk of an 

 embryo at the time of the first appearance of the medullary gi-oove x 300. 

 It shows two large nuclei of the yolk iii) and the protoplasmic network in 

 the yolk between them ; the network is seen to be closer round the nuclei 

 than m the intervening space. There are no areas representino- cells 

 around the nuclei. ° 



12. Nucleus of the yolk in connection with tlie protoplasmic network 

 hardened in osmic acid. 



13. Portion of posterior end of a blastoderm of stage B, showing the 

 formation of cells around the nuclei of the yolk. 



^ 14. Section through part of a voung Scvllium e^g, about r-th of a-^ 

 inch in diameter. 



n. I. protoplasmic network in yolk. 



^.p. zona pellucida. 



ch. structureless chorion. 



/: ep. follicular epithelium. 



X. structureless membrane external to this. 



