H74 



EMBRYOLOGY. 



the communication between it and the future lateral ventricle persists as the fora- 

 men of Monro (Fig. 725, 11). 



_ (Terminal cell. 



Neuroltliixf. 



Nuclei of 

 spongioblasts. 



'J 



Myelospongium 

 network. 





FIG. 721. Transverse section of the spinal cord of a human embryo at the beginning of the fourth week. 

 (After His.) The top of the figure corresponds to the lining of the central canal. 



The second vesicle (ihalamencephalon) becomes elongated from before back- 

 ward and compressed laterally so as to form the greater part of the third 



> s* ^ 



^5T/ 



^/ 



Posterior nerve root. 



j* Central canal. 



Nuclei of spongio- 

 blasts. 



'euroblasts. 



Processes of nenroblasts 

 growing oat to form 

 anterior nerve root. 



^Anterior column. 



FIG. 722. Section of spinal cord of a four weeks' embryo. (His.) 



ventricle (Fig. 725, B). From each side of that part of the forebrain which 

 ultimately becomes the second vesicle is budded off a hollow projection, the 

 primary optic vesicle, which is developed eventually into optic nerve and 

 retina : it will be considered later on. The constriction between the first and 

 second vesicle disappears, so as to throw the whole of the cavity (the future third 



