136 AETHUR DENDY. 



As this is a point of some importance it may be well 

 to enumerate all the cases observed belonging to Stages N, O, 

 and P. 



rEmbryo 93 "^ 



,, 95 >■ Parietal eye lying to left of stalk. 



I 96 J 



Stage N .j ''^ y^ Parietal eye lying directly in front of stalk. 



I „ 97 Parietal eye lying pretty well in front of stalk, but 

 l__ both to left of middle line. 



r „ 89 ^ 



Stage < " 03 [• Parietal eye lying to left of stalk. 



( ", 103 J 

 Stage P „ 87 Parietal eye lying well to left of middle line, stalk 



indistinct (externally). 



At very early stages in the development, immediately ante- 

 cedent to the first appearance of the primary parietal vesicle, 

 the roof of the fore-brain is in a markedly asymmetrical 

 condition, the left half overlapping the right for some distance. 

 This overlap is shown in fig. 1, in the region between the 

 optic vesicles of the ordinary eyes ; it also extends to the 

 region in which the primary parietal vesicle will shortly 

 appear. It will be found more fully illustrated in my memoir on 

 the general development of Sphenodon. Granting that the 

 right parietal eye was once well developed, it seems not 

 unnatural to associate its suppression with this remarkable 

 overlapping of the left side of the roof of the fore-brain, which 

 at first seemed to me to be due to restraint exercised by the 

 very early developed pro-amnion upon the growing brain; but 

 the probability of this explanation is greatly diminished by 

 the fact that a very similar asymmetry of the developing 

 parietal organs occurs in fishes, as described by Hill (16, 17). 

 The part of the brain roof from which the right parietal eye 

 should arise is covered over at first by the overlap, which may 

 perhaps be supposed to retard its development. 



If the right parietal eye ever develops at all it must be 

 represented by the parietal stalk, and there is certainly good 

 ground for considering this to be the case. In the first place 

 the parietal stalk originates^ if I am right in my interpretation 



