THE PARIETAL EYE OF THE LIMULUS. 



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As the embryo develops, this layer of ectoderm extends forward, carrying 

 the ocelh with it and drawing out the ocellar sac into a long epithelial tube or 

 epiphysis. The procephahc ectoderm then forms a vertical wall covering the 

 median anterior surface of the egg; still later it is bent backward onto 



Fig. 93 . — Various forms of retinophorce, isolated by maceration and showing the position and shape of the retinal 

 rods. Cross- sections of the rods are shown over each figure, the place where the section is taken being indicated by 

 the letter S. i . Upright terminal rod from ocellus V of Acilius ; 2 , horizontal terminal rod from sides of ocellus // of 

 Acilius; 3, a giant retinal cell with short horizontal rod, from ocellus //; 4, retinal cell, with lateral rod from com- 

 pound eye of Limulus; s, retinula cell from the compound eye of Tabanus; 6, retinal cell from the ocellus of Lycosa; 

 7, retinula cell, with serrated rod, from the compound eye of Pinseus; 8, inverted retinal cell from the eye of Pecten; 

 9, rod cell from retina of an amphibian (species of Diemyctylus) , showing two nuclei, n'and n, and indications of 

 division of rod into two parts with either a canal or fiber running through a part of the rod; 10, cone cell from 

 same animal, showing double nature of the cell as well as of the cone. The body corresponding to the second 

 nucleus lies at n' 



the haemal surface of the buckler, where it represents the exposed surface 

 of the anterior end of the primitive head, or procephalon; the posterior end 

 of the head, coincident with the forebrain, remaining on the neural surface. 

 (Figs. 152, 153, pc.c.) 



