302 THE CENTRAL NERVOUS SYSTEM. 



strip becomes first detached from the adjoining epiblast, which 

 then meets and forms a continuous layer above it (fig. 190 A 

 and B ;//). The sides of the medullary plate, which is thus shut 

 off from the surface, bend over and meet so as to convert the 



FIG. 190. SECTIONS OF AN AMPHIOXUS EMBRYO AT THREE 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. 



up. neural plate; nc. neural canal; mes. archenteron in A and B, and mesenteron 

 in'C; ch. notochord; so. mesoblastic somite. 



plate into a canal (fig. 190 C nc). 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 epi- 

 dermic layer of the skin is involuted into this keel in Petromy- 

 zon, and Calberla maintains the same view for Teleostei (fig. 

 32), but further observations on this subject are required. In 

 the Teleostei a very shallow depression along the axis of the 

 keel is the only indication of the medullary groove of other 

 forms. 



In Amphioxus (fig. 190), the Tunicata, Petromyzon (?), Elas- 

 mobranchii (fig. 182), the Urodela and Mammalia (fig. 187), the 

 epiblast of the medullary plate is only formed of a single row of 

 cells at the time when the formation of the central nervous 

 system commences; but, except in Amphioxus and the Tuni- 



