THE NERVOUS SYSTEM. 



125 



stage of development, and plays an important part, especially 

 in the younger stages, in determining the shape and proportions 

 of the embryo. 



The epiblast consists almost from the first of two layers, the 

 distinction between which is already established at the close of 

 segmentation. (Fig. 23). Of these the upper or epidermic 

 layer is a single stratum of closely fitting cubical cells ; while 

 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 difficult to see.. 

 {cf. Fig. 2Q). 



H s N 



Fig. 28. Longitudinal vertical section through a frog embryo shortly 

 before the closure of the blastopore. Length of the embryo 2'5 mm. 

 X 30. 



B, blastopore : BF, fore-brain : BH. hind-brain : BM, mid-brain : 

 H, hypoblast : L, liver : M, mesoblast : MN, mesenteron : N, notochord : 

 NC, neurenteric canal : P, ingrowth of epiblast to form pituitary body : 

 PD, proctodajum : R, rectal diverticulum of mesenteron : S, central canal 

 of spinal cord : Y, yolk cells. 



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 



