DEVELOPMENT OF ELASMOBRANCH FISHES. lol 



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 hssmal 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 

 adjoining sheath, but is nevertheless the rudiment of vertebral 

 bodies. PI. xii. fig. 9, vh. 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. 

 Observations on the base of the brain afford fairly conclusive 

 evidence that the homologous tissue present there has this 

 origin. Gegenbaur 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) \ 



The layer of tissue which forms the vertebral bodies rapidly 

 increases in thickness, and very soon, at a somewhat earlier 

 period than represented in Gegenbaur's Plate xxil. fig. 4, 

 a distinct membrane (Kolliker's Membrana Elastica Externa) 

 may easily be recognised surrounding it and separating it 



1 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 figiu*es alluded to. Gotte's very valuable researches 

 with reference to the origin of this layer in Amphibians" tend to confirm the 

 view advocated in the text. 



