THE EYES OF PERIPATUS 115 



cephalic region, is enveloped in an amnion fold which grows up over the 

 ventral plate and becomes cut off from the superficial layer of the amnion 

 fold or serosa, in the same way as in vertebrates, but on the ventral aspect 

 of the body instead of on the dorsal aspect. Another interesting point 

 is that in the scorpion the covering of the dorsal surface of the embryo 

 with skin is effected by the lateral growth of the ventral plate, its right and 

 left edges carrying with them the lines of origin of the amnion on each 

 side, round the dorsal aspect of the ovum, until they meet in the median 

 line. The further study of the development of arthropods, more especially 



Fig. 77. — Ventral View of Three Stages in the Development of Hydrophilus. 



(From Lang, after Heider.) 



a. and b. : points at which the blastopore first closes. 



af. : edge of the amnion fold. 



am. c. : rudiment of amnion cavity commencing at caudal end of embryo. 



c.l. : procephalic lobes. 



g. gr. : linear groove marking the line of invagination of gastral endoderm. 



hf. : head fold of amnion. 



so. : somites. 



of spiders, scorpions, and Limulus, emphasizes the importance of our 

 knowledge of their relation to each other and to their remote ancestors 

 of the Silurian epoch, such as Hemiaspis, and still further back to the 

 ancestors of the Trilobites. The whole subject is, however, very intricate, 

 and it will be impracticable to discuss here the problem of the origin of 

 vertebrates without diverging from the main subject of this part of our 

 treatise, namely an inquiry into the origin and nature of the median eyes 

 of vertebrates. 



It will suffice to state here that the study of the median eyes of inverte- 

 brates does throw light on this question and also that the solution of the 



