278 DESCRIPTION OF THE PLATES. 



k. The roots of the vagus nerve or pneumogastric. The glossopharyngeal 

 nerve forms the most anterior of these strands which rise close 

 together from the medulla oblongata. 



FIG. 4. Longitudinal section through an Elasmobranch embryo at a time when the neural 

 canal, notochord, and alimentary canal are established. From Balfour, Development of Elasmo- 

 branch Fishes. London, 1878, Fig. I c, p. 58. 



ep. Epiblast or superficial layer of cells formed from the blastoderm, and 

 extending, with the mesoblast and hypoblast, round the ovum, to 

 form the yolk sac. This growth or extension is greatest at the anterior 

 end of the embryo. It is slight at the posterior end, where was 

 situated the aperture of invagination, blastopore, or anus of Rusconi, 

 beneath the embryonic rim. 



nc. Neural canal which is formed first as a groove in the blastoderm 

 (medullary groove), and is then converted into a canal by the closure 

 of the sides of this groove. The medullary groove extends to the 

 posterior end of the blastoderm and the dorsal edge of the blastopore. 

 On its closure to form the neural canal a pore persists at this spot ; 

 and when the dorsal edge of this pore, continuous with the edge of 

 the blastopore, grows down over the yolk, a communication is left 

 between the neural and alimentary canals. 



x. Communication between the two canals named, known as the neuren- 

 teric canal. It is a common feature in vertebrate development, and 

 lies, as a rule, behind the spot where the anus is formed. Hence 

 there is a post-anal extension, not only of the neural, but also of the 

 alimentary canal. See Balfour, Comp. Embryology, vol. ii. p. 267, 

 268 ; p. 634-636. Cf. Spencer, Q. J. M. Suppl. 1885, p. 126-7. 



ch. Notochord. A rod of cells differentiated at an early period from the 

 layer of cells below the medullary groove, and therefore probably to 

 be considered as of hypoblastic origin. 



m. Mesoblast. 



al. Alimentary canal, roofed in by cells which are derived together with 

 the mesoblast from the original mass of cells underlying the epiblast. 

 The mesoblast cells are wanting immediately below the medullary 

 groove, but form two plates to either side of it. Both mesoblast and 

 hypoblast extend with the epiblast round the non-segmented part of 

 the ovum or yolk to form the yolk sac. 



n. Nuclei in the yolk. These nuclei appear spontaneously (?) in the pro- 

 toplasm of the yolk, i. e. in the network between the secondary yolk or 

 deutoplasm. A portion of protoplasm separates round each nucleus. 

 This process begins at an early stage, and the cells thus formed are 

 added at first to the lower layer of blastoderm cells, and subsequently 



