STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



The structure of the gelatinous body of the notochord has 

 undergone no important change. The sheath, however, exhibits 

 certain features which deserve careful description. In the first 

 place the attention of the observer is at once struck by the fact 

 that, in the vertebral regions, the sheath is much thicker ('014 

 millim.) than in the intervertebral ('005 millim.), and a careful 

 examination of the sheath in longitudinal sections shews that 

 the thickening is due to the special differentiation of a superficial 

 part (Plate 41, fig. 69, s/i.~) of the sheath in each vertebral region. 

 This part is somewhat granular as compared to the remainder, 

 especially in longitudinal sections. It forms a cylinder (the walj 

 of which is about *oi millim. thick) in each vertebral region, 

 immediately within the membrana elastica externa. Between 

 it and the gelatinous tissue of the notochord within there is a 

 very thin unmodified portion of the sheath, which is continuous 

 with the thinner intervertebral parts of the sheath. This part of 

 the sheath is faintly, but at the same time distinctly, concentri- 

 cally striated a probable indication of concentric fibres. The 

 inner unmodified layer of the sheath has the appearance in 

 transverse sections through the vertebral regions of an inner 

 membrane, and may perhaps be Kolliker's "membrana elastica 

 interna." 



We are not aware that any similar modification of the sheath 

 has been described in other forms. 



The whole sheath is still invested by a very distinct mem- 

 brana elastica externa (m.e/). 



The changes which have taken place in the parts which form 

 the permanent vertebrae will be best understood from Plate 41, 

 figs. 69 71. From the transverse section (fig. 70) it will be 

 seen that there are still neural and haemal arches resting upon 

 the membrana elastica externa ; but longitudinal sections (fig. 69) 

 shew that laterally these arches join a cartilaginous tube, embrac- 

 ing the intervertebral regions of the notochord, and continuous 

 from one vertebra to the next. 



It will be convenient to treat separately the neural arches, 

 the haemal arches with their appendages, and the intervertebral 

 cartilaginous rings. 



The neural arches, except in the fact of embracing a relatively 

 smaller part of the neural tube than in the earlier stage, do not 



