250 THE CENTRAL NERVOUS SYSTEM. 



The fact of the epiblast of the neural canal being divided, like 

 the remainder of the layer, into nervous and epidermic parts, cannot, 

 I think, be used as an argument in favour of the opposite view to 

 that here maintained. It seems probable that the central canal of 

 the nervous system arose phylogenetically as an involution from the 

 exterior, and that the epidermis lining it is merely part of the 

 original epidermis, which has retained its primitive structure as a 

 simple stratum, but is naturally distinguishable from the nervous 

 structures adjacent to it. 



Where the epiblast is divided at an early period into two strata, 

 the nervous stratum is always the active one, and takes the main 

 share in forming all the organs derived from the layer. 



Formation of the central nervous system. In all Chordata an 

 axial strip of the dorsal epiblast, extending from the lip of the 

 blastopore to the anterior extremity of the head, and known as the 



TIC 



FIG. 190. SECTIONS OF AN AMPHIOXUS EMBRYO AT TIIHEE STAGES. (After Kowalevsky.) 



A. Section at gastrula stage. 



B. Section of an embryo slightly younger than that represented in fig. 169 D. 



C. Section through the anterior part of an embryo at the stage represented in 

 fig. 169 E. 



np. neural plate; iic. neural canal; mes. archenteron in A and B, and mesenteron 

 iuC; t-h. uotochord; so. mesoblastic somite. 



medullary plate, becomes isolated from the remainder of the layer to 

 give rise to the central nervous axis. 



According to the manner in which this takes place, three types 

 may, however, be distinguished. In Amphioxus the axial strip be- 

 comes first detached from the adjoining epiblast, which then meets 

 and forms a continuous layer above it (fig. 190 A and B np). The 

 sides of the medullary plate, which is thus shut off from the surface, 

 bend over and meet so as to convert the plate into a canal (fig. 190 C 

 we). In the second and ordinary type the sides of the medullary 

 plate fold over and meet so as to form a canal before the plate 

 becomes isolated from the external epiblast. 



The third type is characteristic of Lepidosteus, Teleostei, and 

 Petromyzon. Here the axial plate becomes narrowed in such a 

 way that it forms a solid keel-like projection towards the ventral 

 surface (fig. 191 Me}. This keel subsequently becomes separated 

 from the remainder of the epidermis, and a central canal is after- 

 wards developed in it. Calberla and Scott hold that the epidermic 

 layer of the skin is involuted into this keel in Petromyzon, and 



