ZOOLOGY 



SECT. 



rim towards the future dorsal and anterior side : this is the 

 blastopore (blp.). The megameres become gradually enclosed by 

 the micromeres as a result of a process which is partly invagina- 

 tion, partly epiboly. During this process the segmentation-cavity 

 becomes displaced by the archenfceron. The dorsal and ventral walls 

 of the latter differ widely from one another, the ventral wall being 

 composed of a thick mass of yolk-cells (megameres) while the roof 

 is comparatively thin and consists of two or three layers of rounded 

 cells. The lumen is a narrow, dorso-ventrally compressed cleft. 

 When the process of gastrulation is completed, the blastopore 

 takes up a position at the postero-dorsal end. The development 

 of the central nervous system differs widely from the corresponding 

 process in Amphioxus, and is only approached among the Craniata 

 by the Teleostomi or Bony Fishes. The dorsal surface becomes 



nc 



ent 



end 



Fia. 804. Fetromyzon. Sections of embryos. A, transverse section of the trunk-region. 

 B, transverse ection of the head-region. c&. ccelomic sacs ; ect. ectoderm, end. endoderm ; 

 ent. enteric cavity ; so. mesoderm-strand ; m.c, mi: medullary keel and medullary cord ; nc. 

 notochord. (From O. Hertwig ; A, after Goette, B, after Kupffer.) 



flattened along a narrow longitudinal area and along this a groove 

 appears, which stops short just in front of the blastopore. The 

 area along which the groove runs soon becomes raised up above 

 the general surface so as to form a narrow longitudinal elevation. 

 Sections of this stage show that the ectoderm has developed a 

 thickening along the course of the longitudinal groove, and this 

 comes to grow downwards towards the archenteron as a solid 

 longitudinal medullary keel (Fig. 803, C, k ; Fig. 804, A, mk). This 

 is the rudiment of the central nervous system. Subsequently the 

 keel becomes separated off from the surface ectoderm, and lies below 

 it as a solid cord. It is only at a considerably later period that a 

 lumen appears in this cord, and gives rise to the ventricles of the 

 brain and the central canal of the spinal cord. During the formation 

 of the medullary keel the rudiment of the notochord is developed 

 from the underlying endoderm very much as in Amphioxus (p. 59). 

 On each side of the medullary cord and notochord is a group of 



