

THE STRUCTURE OF THE MEDULLARY CANAL. 



71 



it is in contact with the overlying ectoderm, from which it has, however, completely 

 separated, and it causes the overlying ectoderm to rise up somewhat. Its sides are 

 in contact with the mesoderm, which is there developing into the primitive seg- 

 ments, page 84. The nuclei in the wall of the canal are very numerous, oval in 

 form, and usually with a single nucleolus. The nuclei are placed in the radial 

 lines. For some time after the canal has become closed the nuclei multiply very 

 rapidly by indirect division, but all of the mitotic figures are found close to the 

 inner surface of the canal, which surface, it will be remembered, corresponds to 

 the original outer surface of the ectoderm. 



Md Seg 



Cho, 



FIG. 38. TRANSVERSE SECTION OF A RABBIT EMBRYO OF EIGHT DAYS AND Two HOURS. 



Md, Medullary canal. Seg, Primitive segments. Cho, Chorion. Am, Amnion. Som, Somatopleure. C<e, 

 Coelom. Spl, Splanchnopleure. Ent, Entoderm. Ch, Notochord. Ao, Aorta. 



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 pro- 

 gresses and the canal closes, these expansions become more marked and are them- 

 selves, 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 (Figs. 129 and 130). 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 expansion 

 becomes unequal, and the inequalities are such that they produce three dilatations, 

 which are known as the three primary cerebral vesicles (Fig. 131). 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 rrfedullary tube. Between the first and second vesicles there is a Con- 



