116 DEVELOPMENT OF THE EROG 



the lower or nervous layer consists of ovoid or spherical cells, 

 more loosely compacted, and two or three deep. It is from the 

 latter that the nervous system is developed. 



The first trace of the nervous system is seen about a week 

 after fertilisation, when the embryo is still spherical and the 

 blastopore has become much reduced in size and diflScult to see. 

 {Gf. Fig. 26.) 



The dorsal surface of the embryo now flattens slightly, and 

 along the flattened area the nervous layer of the epiblast 

 thickens to form the neural plate, which is wide in front but 



EF 



-EN 



Fig. 26. — Transverse section through a frog embryo during the for- 

 mation of the neural canal. . 



C, coelom ; EE, epidermic layer of epiblast ; EN, nervous layer of 

 epiblast ; H, hypoblast ; M, mesoblast ; ME, somatopleuric layer of 

 mesoblast; MH, splanchnopleuric layer of mesoblast ; MN,mesenteron; 

 N, notochord; NF, neural fold ; NG, neural groove ; Y, yolk cells. 



narrows posteriorly towards the blastopore. Slightly raised 

 ridges, the neural folds, soon appear, bordering the sides of the 

 neural plate ; and a longitudinal neural groove is formed along 

 its dorsal surface in the median line, extending forwards from 

 the blastopore. 



A transverse fold connects the anterior ends of the neural 

 folds together, slightly raising up the anterior end of the neural 

 plate. The neural folds now grow rapidly : the groove between 



