THE EMBRYO. 



1177 



The lateral walls of the medullary canal become thickened and marked off 

 into two laminae : a dorsal, or alar lamina^ and a ventral, or basal lamina, the 

 portion of the canal in the mid-line both on its dorsal and ventral surfaces 

 remaining thin, and form the roof and floor plates respectively (Fig. 724). In the 

 thickened lateral portion the neuroblasts begin to collect into groups ; one 

 especially being noticeable in the basal lamina at the situation of the future 

 anterior horn. The processes of this group of cells pass out of the cord and form 

 the anterior nerve-roots : outside this group of cells is the reticulated tissue of 

 the myelospongium, which represents the white matter at this stage, and through 

 which these processes pass obliquely before they leave the cord (Fig. 722). The 

 anterior and posterior columns make their appearance soon after, and as the cornua 



Optic thalamus. 

 Foramen of Monro. 



Posterior commissure. 



<ritit'nl body. 



Cms cerebri. 



Aqueduct of Sylvius. 



Corpora quadrigemina. 

 i Cerebellum. 



IV. Ventricle. 



Cerebral hemisphere. \ 



Rhinencephalon. \ 



Lamina cinerea. 



Optic nerve, i J 

 Optic commissure'. \ 



Pituitary body, j 

 Infundibulum. 



Corpus 

 albicans. 



Spinal cord. 



FIG. 726. Median section of brain of human fostus during the third month. (After His.) 



of gray matter grow- out from the central mass the fissures begin to appear. 

 The anterior fissure is a cleft left between the lateral halves of the cord. The 

 mode of formation of the posterior fissure is uncertain ; many believe that it 

 is a portion of the neural canal, which, dividing into two, forms an anterior part, 

 the permanent canal, and a posterior portion, which becomes filled with a 

 septum of connective tissue from the pia mater, and forms the posterior fissure of 

 the cord. Others are of opinion that it is developed independently of the central 

 canal, as a cleft formed by the enlargement of the lateral halves of the cord, 

 into which an ingrowth of connective tissue from the pia mater takes place. 



At first the foetal spinal cord occupies the whole length of the spinal canal, 

 but after the fourth month the spinal column begins to grow in length more 

 rapidly than the cord, so that the latter no longer occupies the lower part of the 

 canal. 



The ventricles of the encephalon are developed in the manner above described 

 from the five secondary vesicles into which the primary expansion of the anterior 

 extremity of the medullary tube is differentiated. 



