PLATE 13. 377 



vessel, var. Vertebral arch. vb. Vertebral body. vcau. Caudal vein. viii. In- 

 testinal branch of the vagus, v op. Ramus ophthalmicus of the fifth nerve, x. Sub- 

 notochordal rod. 



Fig. i. Section through the anterior part of an embryo of Scy Ilium canicula 

 during stage L. 



c. Peculiar large cells which are found at the dorsal part of the spinal cord. 

 Sympathetic ganglion shewn at syg. Zeiss A, ocul. i. 



Fig. 2. Section through the lateral line at the time of its first formation. 

 The cells marked ;/ / were not sufficiently distinct to make it quite certain that 

 they really formed part of the lateral nerve. Zeiss B, ocul. 2. 



Figs. 3 a, 3/>, 3^, ^d. Four sections of the lateral line from an embryo belonging 

 to stage L. $a is the most anterior. In p,a the lateral nerve (///) is seen to lie in the 

 mesoblast at some little distance from the lateral line. In 3 b and 3 c it lies in 

 immediate contact with and partly enclosed by the modified epiblast cells of the 

 lateral line. In 3 d, the hindermost section, the lateral line is much larger than in 

 the other sections, but no trace is present of the lateral nerve. The sections were 

 taken from the following slides of my series of the embryo (the series commencing at 

 the tail end) $d (46), $c (64), 3 b (84), 3 a (93). The figures all drawn on the same 

 scale, but $a is not from the same side of the body as the other sections. 



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 haemal 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 (:) the separation of the 

 two by the membrana elastica externa 1 (i cl). II: shews also the differentiation of 

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



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

 been much exaggerated by the engraver. 



B. 



