THE ORIGIN OF NERVES. 



291 



physiological language, to undergo the process of "segmenta- 

 tion." At length the division of the germ ceases, and the 

 " blastoderm," or "germinal membrane," is formed. From 



FIG. 53. First stages in the development of a vertebrate animal. Figs. A, B, c, and 

 r> exhibit the process known as "segmentation," or division of the ovum. At E, 

 the germ is seen in a more advanced stage, the primitive groove (e) being repre- 

 sented; whilst in F, the development has advanced still further, d being the 

 primitive groove ; e, the primitive brain ; and./", the first traces of vertebras. 



this latter structure all the parts of the young animal are 

 formed, and the blastoderm itself divides into three layers 

 (Fig. 54), respectively named in the order in which they 

 occur from without inwards the " epiblast " (E), " meso- 



FIG. 54. Development of chick. Magnified cross-section, showing epiblast (E), meso- 

 blast (M), and hypoblast (H) ; a, primitive groove, closing at d d; and c, spinal 

 canal ; ck, the notochord, or early representative of the spine. 



blast" (M), and "hypoblast" (H). Now appears the first 

 trace of the future animal, in the shape of a furrow known 

 as the " primitive groove " (Figs. 53, E, c, and 55, c\ and which 



