372 DEVELOPMENT OF ELASMOBRANCH FISHES. 



plates. This segmentation soon disappears, and between stages 

 K and L the tissue of the vertebral column forms a continuous 

 investment of the notochord which cannot be distinguished from 

 the adjoining connective tissue. Immediately surrounding the 

 notochord a layer formed of a single row of cells may be ob- 

 served, which is not however very distinctly marked 1 . 



During the stage L there appear four special concentrations 

 of mesoblastic tissue adjoining the notochord, two of them 

 dorsal and two of them ventral. They are not segmented, and 

 form four ridges seated on the sides of the notochord. They 

 are united with each other by a delicate layer of tissue, and 

 constitute the rudiments of the neural and haemal arches. In 

 longitudinal sections of stage L special concentrated wedge- 

 shaped masses of tissue are to be seen between the muscle- 

 plates, which must not be confused with these rudiments. 

 Immediately around the notochord the delicate investment of 

 cells previously mentioned, is still present. 



The rudiments of the arches increase in size and distinct- 

 ness in the succeeding stages, and by stage N have unques- 

 tionably assumed the constitution of embryonic cartilage. In 

 the meantime there has appeared surrounding the sheath of 

 the notochord a well-marked layer of tissue which stains deeply 

 with haematoxylin, and with the highest power may be observed 

 to contain flattened nuclei. It is barely thicker than the ad- 

 joining sheath, but is nevertheless the rudiment of the vertebral 

 bodies. PI. 13, fig. 9, vb. Whence does this layer arise? To 

 this question I cannot give a quite satisfactory answer. It is 

 natural to conclude that it is derived from the previously existing 

 mesoblastic investment of the notochord, but in the case of the 

 vertebral column I have not been able to prove this. Observa- 

 tions on the base of the brain afford fairly conclusive evidence 

 that the homologous tissue present there has this origin. Gegen- 

 baur apparently answers the question of the origin of this layer 

 in the way suggested above, and gives a figure in support of his 

 conclusion (PI. XXII. fig. 3)*. 



1 Vide pp. 356, 357. 



* None of my specimens resembles this figure, and the layer when first formed is 

 in my embryos much thinner than represented by Gegenbaur, and the histological 

 structure of the embryonic cartilage is very different from that of the cartilage in the 



