THE DEVELOPMENT OF LEPIDOSIREN PARADOX A. 445 



ment (i. e. the limiting membrane covering it) becomes less 

 and less distinct, and the mass of protoplasm within becomes 

 clear and transparent, and is converted bodily into the 

 rod. Very soon traces of alternate dim and clear zones 

 appear (fig. L). At first these are only two or three in 

 number, but they increase in number as the rod increases in 

 length. 



In the completely developed rod, as already shown, the 

 alternating dim and clear discs are very numerous, and the 

 limiting membrane is no longer to be detected ensheathing 

 the rod. It is, I believe, quite correct to say that now the 

 whole of the protoplasm at the peripheral end of the visual 

 cell has become converted into a cuticular product, the rod. 



The rods develop first in the region of the axis of the eye, 

 from which region their development gradually spreads 

 outwards. 



Auditory Organ. 



The auditory organ appears as a solid ingrowth of the 

 deep layer of epiblast. A split is already present almost 

 from the first in its interior, e. g. Stage 21, and about Stage 

 23 this dilates at its inner end into a definite cavity. 



The Peripheral Nerves. 



The development of the peripheral portions of the nervous 

 system is of such interest and importance that I propose to 

 devote a separate paper to its description. I may here simply 

 summarise the chief facts which I have observed in regard 

 to it in L e p i d o s i r e n . 



The first and the all-important fact which has to be men- 

 tioned is that I find that the motor nerve connection between 

 neural rudiment and myotome already exists at a time when 

 these structures are in close apposition. Where the myotome 

 and spinal cord remain in contact in the section it is of course 

 practically impossible to demonstrate the existence of the 

 nervous bridge between them. If, however, such an embryo 

 be examined as that figured on PI. 10, fig. 24, of the first part 



