STUDIES IN THE IIETINA. 351 



No mention was made in that paper of the protomitomic 

 system^ which^ it is obvions, must complicate this migration. 

 The nuclei are not free^ like leucocytes, to wander through 

 the tissues, but are nodes of a reticulum continuous through 

 the whole retina. How can we reconcile the two phenomena ? 

 Before making an attempt it may be well to describe an 

 observation which might possibly throw some light upon the 

 mechanism of this movement. 



In the last section we described two phases in the retinal 

 nuclei which found the readiest explanation in a pulsatory 

 activity of the individual nuclei, an activity which drove, or 

 assisted in driving, fluid outwards through the retina. We 

 described extraordinary expansions of the nuclei, in which the 

 nuclear membrane played a comparatively passive part. We 

 have now to describe an entirely difl^erent kind of nuclear 

 expansion, which may culminate in a complete or partial dis- 

 appearance of the membrane. Now and again, under 

 circumstances the nature of which I have not discovered, 

 a delicate angular network is seen protruding from, or at 

 least attached to, some of the nuclei. The threads of the 

 reticulum, though very delicate, are fairly visible, because 

 apparently slightly coated with staining matter, and at their 

 nodes minute staining microsomes occur. These extra-nuclear 

 networks seem, in the retiuas in which I have found them, to be 

 specially noticeable in the ganglionic layer (see figs. 27 a — c). 

 But fig. 27 d shows a nucleus of the middle layer thrusting a 

 network into the outer reticular layer, and I have already 

 figured two cases in other layers in Part II, figs. 26 and 27. 

 But in none of these figures did I at the time observe any 

 filaments connecting the networks with adjacent nuclei, 

 although such must have existed. In the first set of figures 

 the extra-nuclear network seems to occupy the place of the 

 granular cytoplasm commonly associated with " ganglionic " 

 nuclei. In fig. 27 a, the two nuclei on the extreme right are 

 connected by their extra-nuclear networks. In the earlier 

 observations (figs. 26 and 27, Part II) the networks seemed 

 to be in one case the projection of a nucleus advancing 



