CHANGES BETWEEN 24 AND ^;^ HOURS 55 



optic vesicles are first formed there is no constriction between 

 them and the lateral walls of the prosencephalon, and the 

 lumen of each optic vesicle communicates mesially with the 

 lumen of the prosencephalon without any definite line of 

 demarcation. 



The relation of the notochord to the divisions of the brain is of 

 importance in later developmental processes. The notochord 

 extends anteriorly as far as a depression in the floor of the » 

 prosencephalon known as the infundibulum (Fig. 19). There- 

 fore, the rhombencephalon, mesencephalon, and that part of the - 

 prosencephalon posterior to the infundibulum lie immediately 

 dorsal to the notochord (are epichordal) while the infundibular 

 region and the parts of the prosencephalon cephaHc to. it project 

 anterior to the notochord (are pre-chordal) . 



The Anterior Neuropore. — The closure of the neural folds 

 takes place first near the anterior end of the neural groove and 

 progresses thence both cephalad and caudad. At the extreme 

 anterior end of the brain region closure is delayed. As a result 

 the prosencephalon remains for sometime in communication 

 with the outside through an opening called the anterior neuro- 

 pore. The anterior neuropore is still open in chicks of 27 hours 

 • (Fig. 18). In embryos of 33 hours the neuropore appears much 

 narrowed (Fig. 21). A Httle later it becomes closed but leaves 

 for some time a scar-like fissure in the anterior wall of the 

 prosencephalon (Fig. 23). The anterioif neuropore does not 

 give rise to any definite brain structure. It is important simply 

 as a landmark in brain topography. Long after it has disap- 

 peared as a definite opening the scar left by its closure serves to 

 mark the point originally most anterior in the developing brain. 



The Sinus Rhomboidalis. — The myelencephalic region of the 

 brain merges caudally without any definite line of demarcation 

 into the region of the neural tube destined to become the 

 spinal cord. The neural tube as far caudally as somite forma- * 

 tion has progressed is completely closed and of nearly uniform 

 diameter. Caudal to the most posterior somites the neural 

 groove is still open and the. neural folds diverge to either side of 

 Hensen's node (Fig. 18). In their later growth caudad the 

 neural folds converge toward the mid-line and form the lateral 

 boundaries of an unclosed region at the posterior extremity of 

 the neural tube known because of its shape as the sinus rhom- 



