296 THE RETINA, BY MAX SCHULTZE. 



protoplasm may be associated with the production of the so- 

 called cuticular formations, at least, so far as regards the outer 

 segments, and the refractile bodies of the internal segments.* 

 As in Chickens, previous to hatching, the rods and cones are 

 already developed, though they are then of smaller size than in 

 the adult animal, so is it also at the time of birth in Man and 

 in many Mammals, as, for example, in Ruminants. In the 

 new-born Child and in the Calf the rods and cones are well 

 developed, and divided into internal and external segments, 

 though these are much more slender and shorter than in adults. 

 It is different in the blind litters of Cats and Rabbits ; here, 

 the percipient elements develop subsequent to birth .f Either 

 at the period of birth the limitans externa is still perfectly 

 smooth, or the first indications of rods and cones begin to pro- 

 ject beyond the limiting membrane in the form of rounded 

 elevations ; the formation of distinct rod-like elements follows 

 a few days later, and proceeds as in Fowls, so that the 

 internal segment is first formed, and then the external segment. 

 The first distinctly perceptible laminae of the latter occur at 

 about the fifth or sixth day after birth. At the ninth day, that 

 is to say, at the period when the eyelids open, the length of the 

 external segments in the Cat amounts to scarcely more than 

 four micromillimeters, whilst in the adult animal their length 

 amounts to seventeen micromillimeters. In the Rabbit the 

 proportions are similar.! The laminae consequently increase, 

 not in thickness but in number. At what period antecedent 



* Hensen long held the opinion that the rods, or the outer part of their 

 substance, develop coincidently with the pigment from the outer lamina of 

 the primary eye vesicle (Virchow's Archiv, Band xxx. , p. 181, and Archiv 

 fiir Mikroskop. Anatomie, Band ii., p. 421), but he has lately given up 

 this view (Ibidem, Band iv. , p. 349). 



t Max Schultze, Archiv fiir Mikroskop. Anatomie, Band ii., p. 246, and 

 Band iii., p. 373. Steinlin, Anatomie der Retina, St. Gallen, p. 99. 



J Max Schultze, loc. cit., Band iii., p. 375. 



For a different view, by W. Krause, see his Membrana fenestrata, 

 p. 33. I may here state, that satisfactory conclusions respecting the 

 development of the rods and cones can only be obtained from the borders 

 of folds of absolutely fresh specimens of retinae, preserved in aqueous 

 humour or iodine of serum, and that all my statements rest on the 



