446 ^H- HIIjL, Developmental history of primary segments of the vertebrate head. 



The internal ridge just opposite the external groove g has at its apex 

 a groove g'. These grooves mark the dividing line between neural 

 segments 7 and 8. Compare this with Tig. 46. 



Fig. 46. Horizontal section through the left wall of the medulla 

 of S. purpuratus, 22 days old. X 275 diameters. The external and 

 internal grooves (g and g'), as in Fig. 45, mark the dividing line be- 

 tween neural segments 7 and 8. 



Fig. 47. Transverse section through the head of S. purpuratus^ 

 31 days old. The section passes through the region of the mid-brain 

 and shows on the left side the oculomotor nerve (III) and its con- 

 nection with the brain. 



Fig. 48. Horizontal section through the cerebellum (segment 6) 

 and adjacent segments of S. purpuratus, 23 days old. The right side 

 only is represented, and shows the connection of the anterior root of 

 the right trigeminal nerve with the crest of the cerebellum and the 

 Gasserian ganglion. 



Fig. 49. Horizontal section through the head of S. purpuratus, 

 31 days old. On the left side the section shows the union of the 

 fifth pair of cranial nerves with segment 7 and the seventh and eighth 

 pair with segment 9. 



Fig. 50. Horizontal section through the head of a chick embryo, 

 40 hours old. The figure represents only the right side, and shows 

 the union of the fifth pair of nerves with the posterior portion of seg- 

 ment 7, the union of the seventh and eighth pair with the posterior 

 portion of segment 9, and the union of the ninth pair with segment 11. 



Corrigendum 

 page 421, line 7 from top, for ^'They" read ''The". 



