STRUCTURE AND DEVELOPMENTAL RATE 233 



Specimens have again been recorded from my experiments, 

 and these also may be induced in a great number of ways, show- 

 ing either two poorly formed eyes, cyclopia, or anophthalmia, 

 accompanied by a narrow tubular brain. Here the arrest or 

 slowing in developmental rate has affected the optic outgrowths 

 only slightly in some cases, in other cases severely, but in all 

 cases it has persisted or continued to act for a longer period and 

 has thereby also suppressed the outgrowths which normally form 

 the series of primary bilateral brain ventricles, hence the final 

 narrow tubular brain. Depending, then, upon the rate of devel- 

 opment at a given moment, we may obtain : first, as is normally 

 the case, optic vesicles on a brain with three bilateral primary 

 ventricles; second, no optic vesicles, yet a brain with the bilateral 

 primary ventricles ; in the third case, we may or may not obtain 

 optic vesicles on a brain with no growth of the bilateral ventricles 

 — a simple tubular brain (fig. 31). 



We may describe the development of the central nervous sys- 

 tem in the vertebrate embryo very simply and schematically as 

 follows. At first a more or less straight linear growth takes 

 place until a given length for the given species is attained, then 

 the linear growth possibly becomes slower in rate and lateral 

 branches or outgrowths begin to appear, first the optic vesicles 

 and then the first, second, and third primary brain ventricles. 

 A competitive element is involved in the origin and growth of 

 the lateral outpushings so that should one of these fail to express 

 itself during the usual time for such expression, it is later unable 

 to grow out normally or may not grow out at all (fig. 31). 



We know from experimental demonstration (Lewis, '04; Sper- 

 mann, the writer, Leplat, '19, and others) that the optic vesicles 

 are derived from a definitely located group of cells in the neural 

 plate of the embryo. When they do not arise from this group of 

 cells no other cells are capable of forming optic vesicles and they 

 do not appear at all. 



In addition to our knowledge of this definitely located optic 

 anlage in the embiyonic brain, I have now to contribute a fact of 

 equal importance in the development of the eye which may be 

 stated thus. When the optic vesicle does not grow out from the 



