308 S. WA'J'ASE. 



appearance. In this stage the " dorsal organ " (Z>. 0) is already 

 very prominent. The eye itself does not show its characteristic 

 invagination, but is represented by a slightly thickened patch of 

 the ectoderm ; except in the microscopic examination of the sec- 

 tions it is hardly recognizable. Tiie section Fig. 42, PI. 

 XXXII, shows an embryo considerably more advanced than 

 Fig. 3!). The plane of section passes obliquely forward from 

 behind, and cuts both the eye {^ and the " dorsal organ " 

 {I). (J). Tiie most noticeable feature in this stage of the devel- 

 opment of the eye is the existence of the lateral invaginations at 

 the edges of the ocular area. The section shows three portions, 

 the dorsal fold {d./) and ventral fold {«./), with the median 

 portion {£) between them. The groove along which the 

 ectoderm invaginates is V-shaped, with the pointed end turned 

 towards the posterior end of the body. As the invagination 

 becomes deeper, the pointed end of the V becomes prolonged into 

 a tube, which works its way beneath the general surface of the 

 ectoderm. This tubular invagination of the posterior part of 

 the eye will be called the median fold of involution {m.f) in 

 our subsequent descriptions. 



The whole outline of the invaginated area may be represented 

 by Y, the area intercepted by the two divergent branches of the 



Y being the ommatidial portion, while the posterior stem of the 



Y is the continuation (4' the lateral depressions, the median fold, 

 extending backward and downward beneath the surface of the 

 general ectoderm. 



The opening of the tube is at the junction of the two divergent 

 branches of the Y with the posterior median stem. 



Figs. 47-56, PI. XXXIII, represent a series of transverse 

 sections of the compound eye in the stage represented by Fig. 

 42, PI. XXXII. Fig. 47 is the section passing through the 

 anterior end of the ocular area. The ectodermic cells are con- 

 siderably thicker than the general ectoderm in this region. The 

 section which comes immediately behind this is not given in the 

 plate. P'ig. 48 shows essential characters of the section which 

 immediately follows the section Fig. 47. At two portions of the 

 thickened ectoderm, rf./ and ?;._/, the cells arrange themselves 

 in radial fashion, accompanied by a characteristic change in each 

 nncleus, which becomes coarsely granular and peculiarly trans- 

 lucent. 



