COMPO IJND EYES OF A R THROronS. 309 



In examining the next section, Fig. 49, tlic radial arrange- 

 ment of cells in the above section is shown to be the beginning 

 of invagination. In Fig. 49 the lateral invaginations c?./ and 

 i>./have each a well developed lunion. In the median side the 

 walls of the invaginations become continuous with the group of 

 finely divided cells, each with darkly stained nucleus. This 

 group of cells give rise to omuiatidia. The outer wall of each 

 invagination becomes coutinuuus with tiie coiunuial cpitlielial 

 covering of the body. 



Figs. 50 and 51 show essentially the same thing. Tiie omma- 

 tidial area {Om) in Fig. 51 becomes wider than in any of the 

 preceding sections. 



Just at the point where the maximum width of the omma- 

 tidial area is reached (Fig. 51), it again begins to become narrow, 

 for in the section (Fig. 52) which comes immediately behind that 

 shown in (Fig. 51), the ommatidial area is narrower compared 

 with that shown in Fig. 51, and the two lateral folds of invagi- 

 nations approach each other, resulting in a fusion in the median 

 line in Fig. 53. In Fig. 54 the fused lateral folds, resulting in 

 the formation of the median fold (?«./), begins to work its way 

 beneath the level of the skin. In Fig. 55 a few cells {m. /), 

 forming the posterior extremity of the median fold beneath the 

 skin, are found. The section (Fig. 56) ])assing behind that shown 

 in Fig. 55 shows no trace of invagination of any kind. 



The anterior end of the eye, then, begins with Fig. 47 ; two 

 sections behind it a beginning of lateral invaginations becomes 

 discernible; at Fig. 53 the two lateral folds fuse, and at the 

 ninth section (Fig. 54) they form a median tube (m./), and at 

 Fig. 56 the posterior boundary of the ommatidial area terminates. 

 In Figs. 45a and 46a are shown the longitudinal sections of 

 the eye." The ommatidial cells in the anterior portion of the eye 

 send out nerve fibres which form the optic nerve {Op. n). The 

 basement membrane {B.M) which underlies the ectodermal cells 

 forms a complete sheath to the bundle of optic nerve fibres going 

 to the optic ganglia of tiie brain, which are formed by inde- 

 pendent involutions on the ventral side of the body.' The origin 



' The optic ganglia arise as ectodermio involutions in tlie pre-oral region of 

 the animal. Owing to the existence of a great cephalic flexure in Limulus, that 

 side where the optic ganglia as well as the median eye originate has Cor the sake 

 of convenience been called the " ventral side." 



