CRANGON VULGARIS. 29 



It is the manner in which this intrusion of the meso- 

 derm takes place which forms one of the important objec- 

 tions to my published ideas regarding the development of 

 the compound eye. According to my former account I 

 regarded this layer as insinuating itself into the cavity of 

 invagination, from the walls of which I then believed arose, 

 on the one hand, all of the ommatidial elements beneath the 

 layer of Semper's nuclei, and on the other, the optic gan- 

 glion. But the invagination in question actually takes place 

 on the other or dorsal side, and to have the mesoderm creep 

 into such a cavity involves its breaking through a solid 

 ectodermal wall. I now believe that the invagination re- 

 ferred to plays a part in the formation of the optic gan- 

 glion, while the eye itself arises from a proliferation of the 

 ectodermal cells, and that this mesoderm extends itself be- 

 tween the two thickenings thus produced. With this view 

 other difficulties surrounding my account of the develop- 

 ment of the eye disappear. 



GREEN GLAND. The green gland or anteunal gland 

 must be enumerated among the mesodermal structures. It 

 will be recalled that a patch of mesoderm was described 

 as stretching into the base of the antennae and is shown 

 (figs. 49 and 50) as forming a solid mass without a lumen. 

 In stage If (figs. 60 and 61) a cavity appears in this tis- 

 sue and the cells lining it take a well-marked epithelial 

 character, their boundaries being distinct, while those of 

 the remaining mesoderm retain their primitive character. 

 As yet there is no connection with the external world and 

 the convolutions of the gland are but few, the most con- 

 torted portion being that figured in fig. 61 gg. So far as my 

 sections show, neither at this nor at any other stage does 

 the green gland have any connection with any other cav- 

 ity inside the body, a fact which was once regarded as^af- 

 fording serious difficulties in the way of regarding it as a 

 segmental organ and which may have led Keichenbach into 



