xxii "EXPLANATION OF PLATE 14. 



Fig. 4. Section through lateral line of an embryo of stage P at the point where 

 it is acquiring an opening to the exterior. The peculiar modified cells of its inner- 

 most part deserve to be noticed. Zeiss D, ocul. 2. 



Fig. 5. Mucous canals of the head with branches of the ramus ophthalmicus 

 growing towards them. Stage O. Zeiss A, ocul. 2. 



Fig. 6. Mucous canals of head with branches of the ramus ophthalmicus growing 

 towards them. Stage between O and P. Zeiss aa, ocul. 2. 



Fig. 7. Junction of a nerve and mucous canal. Stage P. Zeiss D, ocul. i. 



Fig. 8. Longitudinal and horizontal section through the muscle-plates and adjoin- 

 ing structures at a stage intermediate between L and M. The section is intended to 

 shew the gradual conversion of the cells of the somatic layer of muscle-plates into 

 muscles. 



Fig. 9. Longitudinal section through the notochord and adjoining parts to shew 

 the first appearance of the cartilaginous notochordal sheath which forms the vertebral 

 centra. Stage N. 



Fig. 10. Transverse section through the tail of an embryo of stage P to shew the 

 coexistence of the rib-process and htemal arches in the first few sections behind the 

 point where the latter appear. Zeiss C, ocul. i . 



Fig. n. Transverse section through the centre of a caudal vertebra of an embryo 

 somewhat older than Q. It shews (i) the similarity between the arch-tissue and the 

 hyaline tissue of the outer layer of the vertebral centrum, and (2) the separation of the 

 two by the membrana elastica externa 1 (met). It shews also the differentiation of 

 three layers in the vertebral centrum : vide p. 374. 



EXPLANATION OF PLATE I 4 . (X. p. 378.) 

 This Plate illustrates the Formation of the Spinal Nerves. 



COMPLETE LIST OF REFERENCE LETTERS. 



a r. Anterior root of a spinal nerve, ch. Notochord. com. Commissure connect- 

 ing the posterior roots of the spinal nerves, i. Mesoblastic investment of spinal cord. 

 m p. Muscle-plate. n. Spinal nerve, n c. Neural canal, p r. Posterior root of a 

 spinal nerve, spg. Ganglion on posterior root of spinal nerve, vr. Vertebral rudi- 

 ment, w. White matter of spinal cord. y. Point where the spinal cord became 

 segmented off from the superjacent epiblast. 



Figs, i, 2, and 3. Three sections of a Pristiurus embryo belonging to stage I. 

 Fig. i passes through the heart, fig. 2 through the anterior part of the dorsal region, 

 fig. 3 through a point slightly behind this. (Zeiss CC, ocul. 2.) In fig. 3 there is 

 visible a slight proliferation of cells from the dorsal summit of the neural canal. In 

 fig. 2 this proliferation definitely constitutes two club-shaped masses of cells (/ r) the 

 rudiments of the posterior nerve-roots, both attached to the dorsal summit of the 

 spinal cord. In fig. i the rudiments of the posterior roots are of considerable length. 



1 The slight difference observable between these two tissues in the arrangement of their nuclei has 

 been much exaggerated by the engraver. 



