50 



EMBRYOLOGY 





longitudinal groove appears on the dorsal surface of either half and divides it into 

 a medial column, the paraxial mesoderm, lying on the side of the neural tube, and 

 a lateral portion, the lateral mesoderm. The mesoderm in the floor of the groove 

 connects the paraxial with the lateral mesoderm and is known as the intermediate 

 cell-mass; in it the genito-urinary organs are developed. The lateral mesoderm 

 splits into two layers, an outer or somatic, which becomes applied to the inner surface 

 of the ectoderm, and with it forms the somatopleure ; and an inner or splanchnic, 

 which adheres to the entoderm, and with it forms the splanchnopleure (Fig. 16). 

 The space between the two layers of the lateral mesoderm is termed the celom. 



THE NEURAL GROOVE AND TUBE. 





In front of the primitive streak two longitudinal ridges, caused by a folding up 

 of the ectoderm, make their appearance, one on either side of the middle line 

 (Fig. 16). These are named the neural folds; they commence some little distance 



Yelk-sac 



Amnion 



Neural groove 



Neurenteric canal 



Primitive streak 

 Body-stalk 



FIG. 17. Human embryo length, 2 mm. Dorsal view, with the amnion laid open. X 30. (After Graf Spee) 



behind the anterior end of the embryonic disk, where they are continuous with 

 each other, and from there gradually extend backward, one on either side of the 

 anterior end of the primitive streak. Between these folds is a shallow median 

 groove, the neural groove (Figs. 16, 17) . The groove gradually deepens as the neural 

 folds become elevated, and ultimately the folds meet and coalesce in the middle line 

 and convert the groove into a closed tube, the neural tube or canal (Fig. 18), the 

 ectodermal wall of which forms the rudiment of the nervous system. After the 

 coalescence of the neural folds over the anterior end of the primitive streak, the 

 blastopore no longer opens on the surface but into the closed canal of the neural 

 tube, and thus a transitory communication, the neurenteric canal, is established 

 between the neural tube and the primitive digestive tube. The coalescence of the 

 neural folds occurs first in the region of the hind-brain, and from there extends 

 forward and backward; toward the end of the third week the front opening (anterior 

 neuropore) of the tube finally closes at the anterior end of the future brain, and 

 forms a recess which is in contact, for a time, with the overlying ectoderm; the 

 hinder part of the neural groove presents for a time a rhomboidal shape, and to this 



