/50 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



between the epiblast and the dorsal angles of the mesoblastic 

 somites. 



The process of formation of the cranial nerves and dorsal 

 roots of the spinal nerves is, it will be seen, essentially the same 

 as that already known in the case of Elasmobranchii, Aves, &c. 

 The nerves arise as outgrowths of a special crest of cells, the 

 neural crest of Marshall, which is placed along the dorsal angle 

 of the cord. The peculiar position of the dorsal roots of the 

 spinal nerves is also very similar to what has been met with in 

 the early stages of these structures by Marshall in Birds 1 , and 

 by one of us in Elasmobranchs 2 . 



In the parietal region a cavity has now appeared in part of 

 the trunk between the splanchnic and somatic layers of the 

 mesoblast (Plate 36, fig. 29, b.c.}, the somatic layer (so.) consist- 

 ing of a single row of columnar cells on the dorsal side, while 

 the remainder of each somite is formed of the splanchnic layer 

 (sf.}. In many of the sections the somatic layer is separated by 

 a considerable interval from the epiblast. 



We have been able to some extent to follow the develop- 

 ment of the segmental duct. The imperfect preservation of our 

 specimens has, as in other instances, rendered the study of the 

 point somewhat difficult, but we believe that the figure represent- 

 ing the development of the duct some way behind its front end 

 (Plate 36, fig. 29) is an accurate representation of what may be 

 seen in a good many of our sections. 



It appears from these sections that the duct (Plate 36, fig. 29, 

 sg.} is developed as a hollow ridge-like outgrowth of the somatic 

 layer of mesoblast, directed towards the epiblast, in which it 

 causes a slight bulging. The cavity of the ridge freely com- 

 municates with the body-cavity. The anterior part of this ridge 

 appears to be formed first. Very soon, in fact, in an older 

 embryo belonging to this stage, the greater part of the groove 

 becomes segmented off as a duct lying between the epiblast and 

 somatic mesoblast (Plate 36, fig. 28, sg.}, while the front end still 

 remains, as we believe, in communication with the body-cavity 

 by an anterior pore. 



1 Journal of A nat and Physio! . Vol. xi. p. 491, plates xx. and xxi. 

 : "Elasmobranch fishes," p. 156, plates 10 and 13. [This edition, p. 378, 

 pi. ii, 14-] 



