264 DR WALTER H. GASKELL. 



formed by a thickening only, without any invagination, while 

 Keichenbach says an obscure invagination does take place at a 

 very early stage. So in the vertebrate eye most observers 

 speak only of a thickening to form the retina, but Gotte's 

 observation points to an invagination of the optic plate at an 

 early stage. So also in the eye of the Pecten, Korschelt and 

 Heider consider that the thickening, by which the retina is 

 formed according to Patten, in reality hides an invagination 

 process by means of which, as Biltschli suggests, an optic 

 vesicle is formed in the usual manner. The retina is formed 

 from the anterior wall of this vesicle, and is therefore inverted. 

 The origin of the inverted retina of the vertebrate eye does 

 not seem to me to present any great difficulty, especially when 

 one takes into consideration the fact that it is just the lateral 

 eyes in which the retina is inverted in the Arachnid group. 

 The inversion is usually regarded as associated with the tubular 

 formation of the vertebrate retina, and it is possible to suppose 

 that the retina became inverted in consequence of the involve- 

 ment of the eye with the gut diverticulum. I do not myself 

 think any such explanation at all likely, because I cannot 

 conceive such a process taking place without a temporary 

 derangement — to say the least of it — of the power of vision, 

 and as I do not believe that evolution was brought about by 

 sudden, startling changes, but by gradual, orderly adaptations, 

 and as I also believe in the paramount importance of the 

 organs of vision for the evolution of all the higher types of the 

 animal kingdom, I must believe that in the evolution from the 

 Arthropod to the Cephalaspid, the lateral eyes remained through- 

 out functional. I therefore, for my own part, would say that 

 the inversion of the retina took place before the complete 

 amalgamation with the gut diverticulum, that, in fact, among the 

 proto-crustacean, proto-arachnid forms there were some suffi- 

 ciently arachnid to have an inverted retina, and at the same 

 time sufficiently crustacean to possess a compound retina, and, 

 therefore, a compound inverted retina after the vertebrate 

 fashion existed in combination with the anterior gut diverticula. 

 Thus when the eye and optic nerve sank into and amalgamated 

 with the gut diverticulum, neither the dioptric apparatus or 

 the nervous arrangements would suffer any alteration, and the 



