THE STRUCTURE OF THE MEDULLARY CANAL. 



69 



so that at a certain stage there is a well-marked medullary groove in the cephalic 

 region, the medullary plate behind that, and the primitive streak at the hind end 

 of the embryo. But when the streak has disappeared, the medullary groove is 

 found to extend the entire length of the embryo. Owing to this peculiarity, it is 

 possible in a single embryo to follow all the principal stages of the formation of 

 the medullary groove by the examination of a series of transverse sections. Such 

 a stage is found in the rabbit at nine days, .or in the chick at from thirty to forty 

 hours of normal incubation (Figs. 129, 130, and 147). 



The Medullary Canal. The medullary 

 groove gradually deepens, its sides rising 

 higher and higher and arching more and 

 more toward one another until the edges 

 meet and coalesce, thus changing the groove 

 into a tube the medullary canal (Figs. 37 

 and 38, Md}. The closure of the groove 

 occurs in the cervical region first, and 

 spreads from there in both directions. As 

 the closure progresses forward it completes 

 the canal in the region of the head. It 

 occurs in such a manner that there is a 

 very small opening, which is the last point 

 to close. This opening seems to be a fixed /, Dorsal furrow, 

 point, occupying always the same relative 

 position in all vertebrates. It is called the 

 anterior neuropore. At this time the caudal 

 end of the medullary groove may be still 

 open widely, forming the so-called rhom- 



boidal sinus, compare figures 129 and 130, and it is the last portion to close. Of 

 the entire length of the primitive canal, about one half is the anlage of the brain, 

 while the other half is the anlage of the spinal cord. In the subsequent develop- 

 ment of the brain the transverse expansion of the canal is most conspicuous, while 

 in the development of the spinal cord the elongation of the canal predominates. 

 The dilatation of the brain begins very early. 



The medullary canal produces the entire central nervous system. Some of the 

 cells from its walls migrate out of the wall itself on either side. These cells pro- 

 duce the ganglia. 



The Structure of the Medullary Canal. 



When the medullary canal is first formed, it tends to present a rounded out- 

 line in transverse section. But its lateral walls being thicker than the wall on the 

 dorsal and ventral sides of the canal, the internal cavity appears somewhat flattened 

 (Fig. 37). On its ventral side it lies against the notochord. On its dorsal surface 



FIG. 36. CROSS-SECTION OF A HUMAN EMBRYO OF 



I .54 MM. 



, Ectoderm, ct, Somatic meso- 

 derm. p, Beginning of the embryonic ccelom. 

 g, Junction of the extra-embryonic somatic and 

 splanchnic mesoderm. df, Splanchnic meso- 

 derm. en, Entoderm. me, Mesoderm. ch, 

 Notochord. (After Count Spec.) 



