EXPLANATION OF PLATE 23. XXXV 



in osmic acid, but the sections have been represented without tinting. G i is most 

 dorsal of the three sections. Camera. (Zeiss CC, ocul. i.) 



tic. neural canal, sp.c. spinal cord. pr. rudiment of posterior root. ar. rudiment 

 of anterior root. mp. muscle- plate, c. connective-tissue cells, ch. notochord. 



PLATE 23. 



Fig. H i. Section through the dorsal region of a Pristiurus-embryo in which the 

 rudimentary external gills are present as very small knobs. Camera. {eiss CC, 

 ocul. 2.) 



The section shews the commencing differentiation of the posterior nerve-rudiment 

 into root (//-), ganglion (sp.g), and nerve (), and also the attachment of the nerve- 

 root to the spinal cord (x). The variations in the size and shape of the cells in the 

 different parts of the nerve-rudiment are completely lost in the figure. 



pr. posterior nerve-root, sp.g. ganglion of posterior root. n. nerve of posterior 

 root. x. attachment of posterior root to spinal cord. -w. white matter of spinal cord. 

 i. mesoblastic investment to the spinal cord. 



Fig. H n. Section through the same embryo as H i. (Zeiss CC, ocul. i.) 



The section contains an anterior root, which takes its origin at a point opposite 

 the interval between two posterior roots. 



The white matter has not been very satisfactorily represented by the artist. 



Figs. I i, I n. Two sections of a Pristittna^catbiyo somewhat older than H. 

 Camera. (Zeiss CC, ocul. i.) 



The connective-tissue cells are omitted. 



Figs. I a, I b, I c. Three isolated cells from the ganglion of one of the posterior 

 roots of the same embryo. 



Figs. K I, K II. Two horizontal longitudinal sections through an embryo in 

 which the external gills have just appeared. K i is the most dorsal of the two 

 sections. Camera. (Zeiss CC, ocul. r.) 



The sections shew the relative positions of the anterior and posterior roots at 

 different levels. 



pr. posterior nerve-rudiment, ar. anterior nerve-rudiment, sp.c. spinal cord. 

 n. c. neural canal, mp. muscle-plate, mp' . first-formed muscles. 



Fig. L. Longitudinal and vertical section through the trunk of a Scyllium- embryo 

 after the external gills have attained their full development. Camera. (Zeiss CC, 

 ocul. i.) 



The embryo was hardened in a mixture of chromic acid and osmic acid. 



The section shews the commissures which dorsally unite the posterior roots, and 

 also the junction of the anterior and posterior roots. The commissures are unfortu- 

 nately not represented in the figure with great accuracy ; their outlines are in nature 

 perfectly regular, and not, as in the figure, notched at the junctions of the cells 

 composing them. Their cells are apparently more or less completely fused, and 

 certainly not nearly so clearly marked as in the figure. The commissures stain very 

 deeply with the mixture of osmic and chromic acid, and form one of the most con- 

 spicuous features in successful longitudinal sections of embryos so hardened. In 

 sections hardened with chromic acid only they cannot be seen with the same facility. 



