EXPLAN \i K>\ 01 ri \i i; 23. xxxv 



I without ' 

 <lois.il ,,f tin- tlin-c Md I. i. ) 



;/.-. neural can.il. ./. :,!. pr. rudim<-: tot. '"'. ru 



of anterior root. ////-. muscle plate. ..iccdU. i/i. notochord. 



I'l AH 23. 



Fig. 11 i. Section through the < -u of a /V/>7/'///7/f -embryo in which the 



rudimentary external gills arc present as very small knobs. Camera. (Zci.s-> 

 ocul. 2.) 



The section shews the commencing differentiation of t r nerve-rudiment 



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

 root to the .spinal cord (x). The variations in the si/.e and .shape of the cell.s 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 p< 

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

 /'. mesoblastic investment to the spinal cord. 



Fig. H II. Section through the same embryo as II 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 a; 



Figs. I I, I II. Two sections of a rristiitrus-embryo somewhat older than II. 

 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 n. Two horizontal longitudinal sections through an emir 

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

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



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, nip. muscle-plate, nip', tirst-fonned muscles. 



Fig. L. Longitudinal and vertical section through the trunk of a Sfyllt urn ^embryo 

 after the external gills have attained their full development. Camera. (2 

 ocul. i.) 



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



The section shews the commissures which dor.s.illy unite the posterior roots, and 

 also the junction of the anterior and posterior roots. The es 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 le>s completely fused, and 

 certainly not nearly so clearly marked as in the figure. The o>: very 



deeply with the mixture of osmic and chromic aci-l. .111 i t.-nn one of the nut con- 

 spicuous features in successful longitudinal .sections of embryos >o hardened. In 

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



