540 E. I. WERBER 



tion, while decreasing posteriorwards, it becomes gradually less 

 and less destructive and only so interferes with the typical 

 physiological reactions, as to diminish the capacity for normal 

 differentiation. Thus in more posterior regions of the head of 

 some teratophthalmic embryos the result is not an elimination 

 of tissue but often a developmental inhibition. 



Such an inhibition manifests itself mainly in the cells of pos- 

 terior parts of the brain. They sometimes remain in the neuro- 

 blast stage in spite of the relatively advanced age of the embryo. 

 This can be observed particularly in embryos in which the 

 malformation has involved more than the eyes only. 



Another observation which points to inhibition is that con- 

 cerning the presence, usually in the mid-brain and more posterior 

 parts of the brain, of many large tissue spaces (fig. 69), which 

 are sometimes clear and empty while often they may be filled 

 with fibrin (figs. 45, 46 and 48). These spaces evidently repre- 

 sent persistent early vascular anlagen which, owing to chemi- 

 cal incapacity, have been unable to develop any further and 

 connect up into continuous vascular channels. The probability 

 of the latter assumption is much enhanced by the fact that the 

 heads of such embryos during life, owing to accumulation of 

 fluid in the tissue spaces, are sometimes greatly distended. 



This intracerebral oedema in the fish embryo, resulting, appar- 

 ently from the absence of a continuous system of haemal and 

 lymphatic drainage, is, since the fish brain has neither lateral 

 ventricles nor a choroid plexus, in a manner comparable to the 

 congenital internal hydrocephalus of man. Such a condition of 

 hydrops is often found to exist also in the cranial cavity which 

 in these cases is unusually distended and sometimes contains 

 fibrin (cf. figs. 46, 48 and 52). The extracerebral oedema is 

 also probably due to blood — or Ijanph vascular imperfections 

 resulting from a developmental arrest. It is comparable to the 

 human congenital external hydrocephalus, i.e., a condition, where 

 an accumulation of fluid is found in the arachnoidal space. 

 Both the extracerebral and the intracerebral oedema may some- 

 times be found in the same embryo (figs. 46 and 48). 



The suggestion would thas seem to be at hand that both 

 forms of hydrocephalus in man may be due to local inhibition 



