DEVELOPMENT OF THE TADPOLE. IJ-O 



has been entirely removed, there appears a distinct Hne of separa- 

 tion between them. Balfour, in his embryology, tells us that the 

 posterior roots of the spinal nerves arise from the dorsal, and the 

 anterior roots of the same from the ventral side of the spinal 

 cord. These anterior and posterior roots, although at first sepa- 

 rate, ultimately coalesce to form the true nerve-cord. 



In sections i, 2, and 3, we see one stage in the development of 

 the heart, lungs, kidney (or pronephros), and liver. The heart is 

 at this period peculiarly interesting, consisting, as it does, of a 

 pear-shaped sac, situated below, and a little to one side of the 

 mouth-cavity, within the pericardium. It is an open tube, bounded 

 by muscular walls, consisting distinctly of two rows of cells placed 

 end to end, so as to give somev/hat the appearance of transparent 

 rows of beads, threaded on an opaque, granular, and elastic 

 string, nearly the same size as the beads themselves. The inner 

 row represents the epithelial lining of the heart, whilst the outer 

 layer gives rise to its muscular wall. The interior of the sac 

 contains several round, granular cells, looking very like undiffer- 

 entiated yelk-cells, but very much smaller and quite round. 

 Attached to the upper portion of the sac, and towards the mouth- 

 cavity, one of the sections shows another sac, formed exactly on 

 the same principles, consisting, indeed, of a double tube, the wall 

 on one side conterminous with the wall of the heart, and the 

 inner tube filled with granular matter. The pear-shaped body 

 has exactly the shape of a ventricle, whilst the other fairly 

 well represents the position of the auricles, but as there is no 

 distinct connection to be traced between this structure and the 

 heart, besides their close approximation, it very probably represents 

 a young stage in the development of the lungs. The granules 

 within the pear-shaped body are distinctly arranged in the form of 

 two lines crossing each other, with little groups scattered about. 

 Between the heart and the undifferentiated yelk-mass lies the 

 liver. The rudiments of this organ make their appearance very 

 early ; indeed, traces of it are to be found in the animals whose 

 sections were given in the last article. 



In sections 2, 3, 4, and 5, may be seen, just under the noto- 

 chord, an apparently granular structure, of which a much-enlarged 

 vertical section is seen in Fig. 8. This is the immature kidney, or 



