454 J. GUAHAM KERU. 



19. The lens also arises by a solid rudiment from the deep 

 layer of the ectoderm, its cavity arising secondarily. 



20. In the histogenesis of the visual layer of the retina the 

 development of the rods is preceded by the formation of the 

 oil globules. 



21. Each oil globule passes with surrounding protoplasm 

 into a pocket-like projection of the external limiting mem- 

 brane. 



22. The rod is formed directly from the protoplasm lying- 

 in the conical apical part of the pocket. 



23. The motor nerve-trunks are in Lcpidosiren already 

 laid down at a period when myotome and neural tube are still 

 in close apposition. As development proceeds and the myo- 

 tome recedes from the spinal cord the nerve-trunk lengthens 

 out_, increases in thickness, and becomes ensheathed in mesen- 

 chymatous protoplasm. 



24. In the earliest stage observed the motor nerve-trunk is 

 seen to be perfectly continuous with the protoplasm of the, at 

 this stage, simple epithelial muscle-cell of the myotome. 



EXPLANATION OF PLATES 25—28, 



Illustrating Professor Graham Kerr's paper on " The Deve- 

 lopment of Lepidosiren paradoxa," Part III. 



Tlic figures of sections have been drawn willi tlie Abbe camera under Zeiss 

 objectives a*, a, d, 3 mm. dry apocbromatic, and ^^r in. homogeneous immer- 

 sion. The figures of tlie entire brain have been done either from actual 

 dissections, or from reconstructions l)y tlie method described liy mc in an 

 earlier number of this Journal.' 



]5y Stage n in tlie descriptions of the following figures I mean the stage re- 

 presented by fig. n in Part 1.^ Stages slightly younger or ohler than the 

 figured are described as n- or n + . 



' ' Quart. Journ. Micr. Sci.,' vol. xlv, p. 5. 

 2 ' rhil. Trans..' vol. cxcii, p. 299. 



