212 patten: [Vol. I. 



nately, not been able to procure the right stages for observing 

 the rapid changes which take place just after the formation of 

 the deep and open pits. Whether the two layers are formed by 

 a delamination of the floor of the optic cups or by their closure, 

 I cannot say. On theoretical grounds a delamination of the 

 walls of an open pit does not seem probable, yet, in the 

 youngest stages of the double-layered condition observed, the 

 two layers appeared as though they had been formed by 

 delamination, rather than by folding. It is possible, however, 

 that this appearance was due to the fact that those stages had 

 been treated only with alcohol, and, consequently, were not well 

 preserved. Be this as it may, the fact I desire to emphasize 

 here, is that the three ocelli arise from four single-layered pits, 

 the median ocellus being formed from a coalescence of the two 

 ventral ones. 



The double nature of the median ocellus is also shown by 

 the fact that, even in the latest stages, the root of the nerve is 

 double, while that of the other two is single. 



My observations, therefore, differ considerably from those of 

 Carriere (21) on the Hymenoptera, since he describes the first 

 stages as disc-like thickenings, which soon split into two layers. 

 On the periphery of the double layer thus formed, is a " schrag 

 nach unten gerichtete Einstiilpung in welche die beiden 

 Schichten in ihrer normalen Lage eingehen." This description 

 corresponds with what I have seen in young pupae of Vespa. 

 But we have seen that the first stages of the ocelli were not 

 to be found in pupae, but in the oldest larvae. Therefore, it is 

 probable that what Carriere regarded as the " anlage" of the 

 ocelli was nothing but the already closed optic cups of the 

 larvae. 



ACILIUS AND OTHER FORMS. 



In my paper on the Eyes of Molluscs and Arthropods I 

 expressed the opinion that the ancestral Arthropod eye was 

 three-layered, composed of the two walls of an optic cup, and 

 an overlying layer of epidermis. In order to maintain this sup- 

 position it is important to show that the eyes of Dytiscns and 

 related forms, described by Grenacher as open cups, and re- 

 garded by him and his followers as the simplest type of Arthro- 



