MUSEUM OF COMPARATIVE ZOOLOGY. 51 



explanation of the method of its formation other than that implied in the 

 allusion to Grenacher's researches on Myriapoda. 



Carriere ('85, p. 178), basing his conclusions principally upon the ap- 

 pearances presented by the ' stemma ' in the pupa of an ant, believes that 

 it is derived from the hypodermis by, — first, a lenticular thickening of 

 the hypodermis produced by an elongation of the hypodermis cells ; and, 

 secondly, by the rearrangement of the latter into two layers, one above 

 the other, of which the outer remains in continuity vfith the permanent 

 hypodermis and constitutes the " vitreous body," while the inner is trans- 

 formed into the retina. The method by which this rearrangement is ac- 

 complished is to be learned a little farther on (p. 189), where he says : 

 " According to my interpretation, therefore, the simple eye (Napfauge) 

 and the compound eye (Facherauge) of the Arthropoda are organs which 

 arise out of like components in a similar manner (through splitting of the 

 hypodermis into two layers), but in their further development diverge 

 from each other in two opposite directions." 



While the authors just quoted agree in believing the retina to be an 

 immediate derivative from the liypodermis, those cited below are at least 

 so far in agreement ,as to hold that the retina is 7iot developed directly 

 from that layer. 



Graber's objection to the view that the retina is derived from the 

 hypodermis was based principally upon its total separation from the 

 hypodermis and its derivatives (pigment-cells and " vitreous body ") by 

 means of the so-called pre-retinal septum or lamella * discovered by him. 

 Combating Grenacher's conclusions, Graber ('79, p. 66) says : If really 

 the pigment-cells were directly continuous vidth the retinal cells, as 

 Grenacher's Fig. 31 (Vespa) makes them, then there would be an unin- 

 terrupted transition from retina to hypodermis, and consequently the 

 typical two-layer " stemma " could be considered as only a modification 

 of the apparently one-layer eye of the Dytiscus larva. " Fine solche 

 directe Verbindung der Retina," he adds, " mit den das Auge umsaumen- 

 den Integumentzellen existirt aber nicht ; Hypodermis, Pigment- und 

 Krystallkorperzellen einerseits und Retina anderseits bilden vielmehr je 

 ein geschlossenes Gauzes fiir sich, indem sich eben zwischen beiden 

 Straten unser praretinales Septum durch und durch zieht, und so vielleicJit 

 auch fur die Zuldssigkeit der Grenadier' schen Theorie heziiglich des hypo- 

 dermalen (wir sagen nicht ectodermatischen) Ursprimgs der Arthropoden- 

 Retina eine schwer za uberivindeude Schraiike bildet." In his resume 

 of the principal results of his paper Graber (p. 88) gives as the second 

 result : " Die Retina des Stemma ist in ihrer ganzen Ausdehnung durch 



* I shall in the future refer to this structure as the "pre-retinal membraiie." 



