STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 751 



This mode of development corresponds in every particular 

 with that observed in Teleostei by Rosenberg and Oellacher. 



The structure of the notochord (nc.) at this stage is very 

 similar to that observed by one of us in Elasmobranchii 1 . The 

 cord is formed of transversely arranged flattened cells, the outer 

 parts of which are vacuolated, while the inner parts are granular, 

 and contain the nuclei. This structure gives rise to the appear- 

 ance in transverse sections of an axial darker area and a periphe- 

 ral lighter portion. 



The hypoblast retains for the most part its earlier constitution, 

 but underneath the notochord, in the trunk, it is somewhat thick- 

 ened, and the cells at the two sides spread in to some extent 

 under the thickened portion (Plate 36, fig. 29, s.nc.). This thick- 

 ening, as is shewn in transverse sections at the stage when the 

 segmental duct becomes separated from the somatic mesoblast 

 (Plate 36, fig. 28, s.nc.), is the commencement of the subnoto- 

 chordal rod. 



The tail end of the embryo still retains its earlier characters. 

 Seventh day after impregnation. Our series of specimens of 

 this stage is very imperfect, and we are only able to call attention 

 to the development of a certain number of organs. 



Our sections clearly establish the fact that the optic vesicles 

 are now hollow processes of the fore-brain. Their outer ends 

 are dilated, and are in contact with the external skin. The 

 formation of the optic cup has not, however, commenced. The 

 nervous layer of the skin adjoining the outer wall of the optic 

 cup is very slightly thickened, constituting the earliest rudiment 

 of the lens. 



In one of our embryos of this day the developing auditory 

 vesicle still has the form of a pit, but in the other it is a closed 

 vesicle, already constricted off from the nervous layer of the 

 epidermis. 



With reference to the development of the excretory duct we 

 cannot add much to what we have already stated in "describing 

 the last stage. 



The duct is considerably dilated anteriorly (Plate 36, fig. 31, 

 .sg-.); but our sections throw no light on the nature of the ab- 

 dominal pore. The posterior part of the duct has still the form 



1 " Elasmobranch Fishes," p. 136, plate u, fig. 10. [This edition, p. 354, pi. 12.] 



