THE STRUCTURE OF THE MEDULLARY CANAL. 



73 



are found close to the inner surface of the canal, which surface, it will be remem- 

 bered, corresponds to the original outer surface of the ectoderm. 



The differentiation of the brain and spinal cord is indicated even during the 

 stage of the medullary groove. The extreme anterior end of the groove is found 

 to widen out so as to produce a pair of lateral expansions. As development 

 progresses and the canal closes, these expansions become more marked and are 

 themselves, of course, also closed over, so that when the canal is completed they 

 appear as lateral diverticula or evaginations of the tube, which are known as the 

 primary optic vesicles. While the vesicles are developing the medullary tube 

 expands in diameter throughout its cranial or anterior half without any notice- 

 able change in the general histological structure of its walls. Very soon the 



Md Seg 



Fig. 28. — Transverse Section of a Rabbit Embryo of Eight Days and Two Hours. 



Md, Medullary canal. Seg, Primitive segments. Cho, Chorion. Am, Amnion. Som, Somatopleure. Cce, 



CcElom. Spl, Splanchnopleure. Enf, Entoderm. Ch, Notochord. Ao, Aorta. 



expansion becomes unequal, and the inequalities are such that they produce 

 three dilatations, which are known as the three primary cerebral vesicles. The 

 first vesicle is in the region of the optic outgrowth ; the second is just behind 

 this, and the third is as long as the first and second combined and merges 

 into the spinal cord. At the time these vesicles become recognizable they 

 occupy about half the entire length of the medullary tube. Between the first 

 and second vesicles there is a constriction, and one also between the second and 

 the third. The three vesicles are the anlages respectively of the fore-brain, 

 mid-brain, and hind-brain. 



In the region of the spinal cord the medullary tube soon becomes somewhat 



