132 



ZOOLOGY 



SECT. 



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 corres- 

 ponding process in Amphioxus, and is only approached among 

 the Craniata by the Teleostomi or Bony Fishes. The dorsal surface 

 becomes 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. 813,- C, k ; Fig. 814, A, mk). This 

 is the rudiment of the central nervous system. Subsequently the 

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



end 



FIG. 814. Fetromyzon. Sections of embryos. A, transverse section of the trunk-region. 

 B, transverse section of the head-region. COR. coelomic sacs ; ect. ectoderm ; end. endoderm ; 

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

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



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. 57). 

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

 cells arranged as a longitudinal strand the mesoderm plates. 

 In the head-region (Fig. 814, B) a number of diverticula from the 

 archenteron coelomic diverticula are given off into these strands : 

 in the trunk region (A) these are absent. The inner portion of the 

 mesoderm on each side becomes divided up into a series of meso- 

 dermal somites or protovertebrae, the lateral part remaining 

 undivided and forming the lateral plate. In this restriction of 

 somite-formation to the part of the mesoderm immediately adjacent 



