170 



CRUSTACEA. 



boundaries of which can be recognised in the adult crystalline cone. 

 The cells in the lower layers of the ommateal column no doubt give 

 rise to the elements of the retinulae, but Nusbaum is inclined to- 

 derive these from the intermediate columns. Claus derives from 

 these columns the anterior and posterior pigment- cells which 

 surround the crystalline cone in Mysis. The observation of these 

 processes is rendered difficult by the early deposition of pigment 

 which is to be found within the rudiment of the eye in two layers, 

 and in a third mesodermal layer below it. 



tsc^r eS 5f^w5 



Fig. 86.— Sections through the compound eye of the American Lobster (Homarus americamis) 

 in three stages of development (after Parker). A, transverse section through the optic lobe 

 in a young stage. B, older stage when the optic rudiment (r) and the optic ganglion (go) are 

 beginning to be separated by the development of a basal membrane (mb). C, cross section 

 of still older stage, c, rudiment of the brain ; go, optic ganglion ; mb, basal membrane ; 

 r, rudiment of the eye (retinogen). 



The account here given of the ontogeny of the eye in Mysis resembles that 

 given by Herrick (No. 48) in connection with Alpheus, and Parker (No. 62) 

 for Homarus. The optic lobes here develop, by proliferation of the ectoderm, 

 into a multilaminar rudiment (Fig. 86 A). (Herrick considers that indifferent 

 elements out of the yolk also contribute to their formation). There then takes- 

 place a separation into a superficial and a lower part (Fig. 86 B and C). The 

 first layer (retinogen) becomes the rudiment of the eye (r), while that of the 

 optic ganglion (go) arises from the cell-mass of the lower layer (gangliogen). In 

 later stages the two layers are separated by a cuticular basal membrane, through 

 which strands of nerve-fibres pass. In the rudiment of the eye itself, which we 

 have called the retinogen, t\w separate ommatidia are said to develop (Herrick), 

 the corneal cells forming into groups of two in the most superficial layer, and 



