170 PAUL WEISS 



wall. At first, all of these cells divide. The growth function at this stage 

 is therefore a volume function. In the cup stage the retina becomes 

 multilayered, with a sharp division into a germinal and a sterile zone. 

 Only the cell layer in contact with the outer surface, corresponding to 

 the ventricular (ependymal) layer of the brain, continues to proliferate, 

 while the cells released into deeper layers differentiate the various ret- 

 inal strata without further multiplication. The source of growth thus 

 has become reduced to a two-dimensional one, causing a marked decline 

 in the relative growth rate taken over the whole organ (e.g. from meas- 

 urements of diameter). Later, the cells of the germinal layer themselves 

 cease to proliferate and transform into sensory cells, a process which 

 starts from the center (macula) and spreads rapidly toward the pe- 

 riphery (ciliary zone) of the retina. Eventually, only the cells at the rim 

 retain residual capacity to multiply. Further growth is then essentially 

 by apposition from this rim ; that is, the growth source has shrunk 

 from planar to linear extension. Meanwhile some of the neuroblasts, 

 though no longer multiplying, grow in size as they sprout nerve proc- 

 esses, which, grouped into plexiform layers, add to the thickness of the 

 retina. During the later stages a gelatinous secretion, supposed to come 

 from cells of both retina and lens, fills the interior with vitreous humor, 

 thereby progressively distending the eyeball. In addition, blood vessels 

 and other mesenchym penetrate into the eye from the surroundings. 



This diversity and complexity of the component processes contribut- 

 ing to eye size makes the search for a single "growth-controlling" prin- 

 ciple appear utterly unrealistic, and comparisons between different eyes 

 on the sole basis of size must be fallacious. To exemplify briefly the 

 direction that an analytical approach to "differential growth" would 

 have to take, I shall select a few of the concrete questions raised by this 

 picture of eye growth and point out their general significance. 



First, what causes the confinement of proliferation to the single 

 cell layer at the convexity of the retina ? The answer is unknown. But a 

 clue lies in the fact that in this and most similar instances the site of pro- 

 liferation corresponds to some prominent geometrical feature of the 

 system, such as a surface or fold or tip or crest. For the cells concerned 

 this means unique physical conditions (e.g. tensions, pressures, unique 

 exposure to surrounding agencies) not shared by the rest of the tissue. 

 In neural tube and retina, for instance, the direct exposure to the fluid 

 of the central canal and ventricular spaces might be the crucial factor. 

 The fact that injury can activate cells of the interior to take part in pro- 



