1J2 WAJLTEE HEAPB. 



backwards ; it proceeds very rapidly^ being at the end of this 

 stage, although open at its immediate anterior end (fig. 6), 

 closed from there posteriorly until the fourth protovertebra is 

 reached, after which point it gradually widens .out into the 

 sinus rhomboidalis (figs. 28 to 33). 



At the close of Stage h a narrow slit-like pore is all that 

 remains open at the anterior end (fig. 20), while posteriorly it 

 is closed as far back as the eighth protovertebra ; and at the 

 end of Stage j the whole groove is converted into a canal until 

 the last, the fourteenth, protovertebra is reached. 



The sinus rhomboidalis is now narrow and shallow (figs. 48 

 and 50). The swelling in the floor at the hind end of the 

 sinus rhomboidalis is caused by the mesoblast of the front end 

 of the primitive streak (figs. 33 and 35 ; 48 and 50). 



When the canal is first formed, its lumen — except in the 

 anterior region which is described below — is a narrow slit and 

 its walls are thicker at the sides than they are dorsally and 

 ventrally (fig. 28) ; soon afterwards, however, during Stage h, 

 the middle portion of the lateral walls thickens still more and 

 projects into the narrow lumen of the canal, thus converting it 

 into an hour-glass form (fig. 29). 



The cells of the cord are much elongated, and their nuclei, in 

 general, oval (fig. 43). 



I may in this place mention there appears to me to be great 

 likelihood of the migration of mesoblast cells into the walls of 

 the medullary canal during Stages h and j. Sections of an 

 embryo belonging to the former stage present strong evidence 

 of this process (fig. 43). Two masses of mesoblast cells are to 

 be seen in very close connection with the lateral walls of the 

 canal in the region of the neck, and from these masses I feel 

 inclined to believe certain cells grow into the tissue of the 

 nervous system. 



As I will show below, these masses of cells are in connection 

 with two blood-vessels, which are in process of formation, and 

 it would appear highly probable that these ingrowing meso- 

 blast cells give rise to the blood-vessels of the spinal cord. 



The Brain. — When the medullary groove first closes in 



