284 PRINCIPLES OF EMBRYOLOGY 



certain lack of precision. There may have been unavoidable alterations 

 in the environmental conditions, amounts of food, temperature, disease, 

 etc., and each weighing will only be accurate within certain observational 

 limits. Thus what we shall have as a basis for setting up the growth curve 

 is not a set of absolutely precise points but rather a zone of greater or 

 less width within which the curve must lie. It will always be possible 

 to fit quite a number of different theoretical curves into such a zone, 

 particularly if we allow ourselves to consider the possibility of a set of 

 growth phases for each of which a new set of constants may be calculated. 

 Thus it is extremely improbable, and in fact does not happen in practice, 

 that a set of empirical observations can suffice to discriminate between the 

 various theoretical possibilities. 



Moreover, it is quite obvious that when dealing with the growth of 

 an entity such as an embryo, we are not confronted with a 'growth' which 

 corresponds to any precise definition. The embryo contains a highly 

 heterogeneous collection of tissues, some of which are growing, probably 

 at difterent rates, while other parts of the embryo, such as the blood 

 plasma, are not growing in any normal sense at all. 



Weiss (1949^) has given a very clear and vivid picture of the type of 

 complexity which is involved, even in the growth of a single organ, such 

 as the eye. He writes : 'The original eye vesicle consists of a certain initial 

 allotment of cells from the embryonic brain 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 retinal 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 measurements of diameter). Later, the cells of the germ- 

 inal layer themselves cease to proliferate and transform into sensory cells, 

 a process which starts from the centre (macula) and spreads rapidly toward 

 the periphery (cihary 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 processes, 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 humour, thereby 



