HEAD-FOLD TO TWELVE SOMITES 



105 



parts, sutura neurochordalis sen ventralis and sutura terminalis 

 anterior.) 



The neuropore question resolves itself into this: What part 

 of the sutura cerebralis anterior is to be called neuropore? As 

 the suture extends from near the infundibulum to the pineal 

 region at least, there is a wide range of choice. However, there 

 is a point in the suture near its dorsal end where the separation 

 of the ectoderm from the neural tube takes place later than 

 elsewhere. This may be regarded as the equivalent of the 

 neuropore. The suture is the site of formation of the lamina 

 terminalis (Chap. VIII). 



op.Ves 



FIG. 60. The head of the same embryo from 

 below x 30. 



a. i. p., Anterior intestinal portal. End'c. s., 

 Endocardial septum. F. G., Fore-gut. Ht., Heart. 

 N'ch. T., Termination of Notochord. op. Ves., 

 Optic vesicle, p. C., Parietal cavity. Pr'a., Pro- 

 amnion. V. o. m., Omphalo-mesenteric vein. 



It will be seen that according to this account most of the 

 primary fore-brain includes no part of the original floor of the 

 neural tube. 



Primary Divisions of the Neural Tube. The neural tube is the 

 primordium of the brain and spinal cord. Its cavity becomes the 

 ventricles of the brain and the central canal of the cord. There 



