DEVELOPMENT OF THE FECUNDATED EGG. 509 



the primitive groove, appears; this serves as the point 

 of origin of the central nervous system (spinal cord and 

 encephalon). In fact, this line is simply a groove (Fig. 145) 

 bounded by two longitudinal ridges of the external layer or 

 fold (epiblasf) of the blastoderm. These two ridges (medul- 

 lary folds, Fig. 145, 3) extend backwards and, by their 

 union, surround the medullary canal. This canal is repre- 

 sented in the adult by the central canal of the spinal cord 

 with the fourth ventricle and the ventricles of the brain (and 

 the aqueduct of Sylvius). The incomplete closure of this 

 medullary groove results in the formation of the fourth ven- 

 tricle. It is generally admitted at 

 the present time that the external 

 layer of the blastoderm (epiblast) 

 forms only the epithelium of the 

 central canal of the spinal cord 

 (and cerebral ventricles, vibratile 

 epithelium, see p. 190), and that the 

 nerve elements originate in a part 

 of the middle layer (mesoblasi) sub- 

 jacent to this epithelium. This view 

 is confirmed by the fact that every- 

 where else the nerve elements are 

 formed at the expense of the inter- 

 mediate layer (mesoblast). 



The upper portion of the medul- 

 lary or neural canal forms the ence- 

 phalic substance ; this part swells 

 out into three vesicles (cerebral 

 vesicles or cells) which are respec- 

 tively named in order, from front to Fi s> us. Orfgi^f the nervous 

 back : the anterior cerebral, the mid- 

 dle cerebral, and the posterior cerebral cells, or the first, 

 second, and third cerebral vesicles. The anterior or first 

 cerebral cell or vesicle is again divided into two portions, the 

 most anterior of which (anterior of the brain), overlaying the 

 second, forms the cerebral hemispheres and corpus callosum, 

 and the posterior (intermediate portion of the brain) forms 

 the thalami optici and the third ventricle (continuation of 

 the medullary canal) : 2. The middle or second cerebral 



* 1, Medullary groove. 2, Inferior enlargement of the medullary groove 

 (rhomboid sinus). 3, Crests or medullary folds (laminae dorsales). 5, Middle 

 and external folds of the blastoderm. 6, Inner fold of the blastoderm (Bischoff ). 



