xxxiv EXPLANATION OF PLATE 22. 



In H i the rudiments of the posterior roots are of considerable length. 



pr. rudiment of posterior roots, nc. neural canal. ;///. muscle-plate, ch. noto- 

 chord. x. subnotochordal rod. ao. dorsal aorta, so. somatopleure. sp. splanchno- 

 pleurc. a/, alimentary canal, ht, heart. 



Fig. C. Section from a JPristiurus-embryo, slightly older than B. Camera. 

 (Zeiss CC, ocul. 2.) The embryo from which this figure was taken was slightly 

 distorted in the process of removal from the blastoderm. 



vr. rudiment of vertebral body. Other reference letters as in previous figures. 



Fig. D a. Section through the dorsal region of a 70r^W0-embryo with three 

 visceral clefts. (Zeiss CC, ocul. 2.) The section shews the formation of the dorsal 

 nerve-rudiments (pr) and of a ventral anterior nerve-rudiment (ar), which at this early 

 stage is not distinctly cellular. 



ar. rudiment of an anterior nerve-root, y. cells left behind on the separation of 

 the external skin from the spinal cord. c. connective-tissue cells springing from the 

 summit of the muscle-plates. Other reference letters as above. 



Fig. D b. Section from dorsal region of a Torpedo-embryo somewhat older than 

 Da. Camera. (Zeiss CC, ocul. 2.) The posterior nerve-rudiment is considerably 

 longer than in fig. D a, and its pedicle of attachment to the spinal cord is thinner. 

 The anterior nerve-rudiment, of which only the edge is present in the section, is 

 distinctly cellular. 



i. mesoblast growing up from vertebral rudiment, sd. segmental duct. 



Fig. D c. Section from a still older Torpedo-embryo. Camera. (Zeiss CC, 

 ocul. 2.) The connective-tissue cells are omitted. The rudiment of the ganglion (g) 

 on the posterior root has appeared. The rudiment of the posterior nerve is much 

 longer than before, and its junction with the spinal cord is difficult to detect. The 

 anterior root is now an elongated cellular structure. 



g. ganglion. 



Fig. D d. Longitudinal and vertical section through a Torpedo-embryo of the 

 same age as D c. 



The section shews the commissures (x) uniting the posterior roots. 



Fig. E a. Section of a Pristiurus-embryo belonging to the second stage. Camera. 

 (Zeiss CC, ocul 2.) The section shews the constriction of the pedicle which attaches 

 the posterior nerve-rudiments to the spinal cord. 



pr. rudiment of posterior nerve- root. nc. neural canal. ;;//. muscle-plate, vr. 

 vertebral rudiment, sd. segmental duct. ch. notochord. so. somatopleure. sp. 

 splanchnopleure. ao. aorta, al. alimentary canal. 



Fig. Eb. Section of a Pristiurus-embryo slightly older than E a. Camera. 

 > 'C, ocul. 2.) The section shews the formation of the anterior nerve-root (ar). 

 ar. rudiment of the anterior nerve-root. 



Fig. F. Section of a Pristiurus-embryo with the rudiments of five visceral clefts. 

 ( 'aim -ra. (Zeiss CC, ocul. 2.) 



The rudiment of the posterior root is seen surrounded by connective-tissue, from 

 which it cannot easily be distinguished. The artist has not been very successful in 

 rendering this fipure. 



.;. Three longitudinal and horizontal sections of an embryo some- 

 wl-,at ..Her than F. The embryo from which these sections were taken was hardened 



