THE STRUCTURE OF THE MEDULLARY CANAL. 71 



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 wlien 

 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. 167, 168, and 173). 



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 medul- 

 lary canal. 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 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, 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 predomi- 

 nates. 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 

 produce 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. 27). On its ventral side it lies against the notochord. On its 

 dorsal surface 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 segments, page 79. 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 



