STUDIRS IN THE RETINA. 37 



only tliickei- but very much more extensive without the 

 appearance of any special formative cells which would 

 account for it.^ If, however, the nuclei of the middle layer, 

 each with more or less cytoplasm trailing behind it, do 

 actually move along fj-om the rim of the retina towards the 

 axis, we can account not only for the gradual thickening of 

 the inner reticular layer as the eye grows, but also for its 

 curious stratification, which is sometimes very striking. 

 The layer reaches its definitive thickness when the eye has 

 ceased to grow and no more nuclei are produced at the rim. 



So far, however, we have only considered the inner 

 reticular layer, but there are two such layers, as there should 

 be if the mechanics of their formation here sketched be 

 correct. If correct, it supplies us also with an explanation 

 of the fact that the two reticular layers are always co- 

 extensive with the region of rod-formation, only appearing 

 where the vesicles are being protruded. A slight difficulty, 

 however, now arises. If these layers are produced by the 

 nuclei travelling down the middle layer from the rim towards 

 the centre, why is not the reticular layer on the outer side of 

 the stream as thick as that on the inner? An answer may 

 be suggested which is probably correct, although it would 

 be difficult to bring any evidence for or against it. Great 

 numbers of the nuclei travelling on the outer side are 

 arrested as they go and ])ass into the layer of rod nuclei. 

 These might be expected to take all the cytoplasm they 

 could with them as the formative substance of the vesicular 

 proti'usious which they are destined to send out from the 

 retina for the formation of their rods. Hence it is probable 

 that the greater part of the cytoplasmic reticulum which 

 Avould otherwise be accumulated here as a counterpart of 

 the inner reticular layer is carried outwards and used up 



' 1 liave never seen any indication of the rows of small, faintly oullincil, 

 formative cells such as Borysiekiewitz (I. c.) describes for the inner reticular 

 hiyer in human retinas ; whenever nuclei do occur in the layer, in ail the eyes 

 1 have examined, they are always quite distinct, and to be regarded as mii^raf- 

 ing outwards from the "ganglionic cell" layer. 



