THE EMBRYOLOGY OP A SCOEPION. 127 



are further forward in relation to the mouthy which can now 

 be seen lying between the bases of the chelse. 



The g-enital opercula begin to grow out from the body wall 

 and the genital duct begins to be formed. This last (PI. XVII, 

 fig. 46) is developed in the mesoblast as a tubular portion of 

 the coelom, but does not open to the exterior up to the time of 

 hatching. It may be nephridial in its nature, but this very 

 late formation of the external aperture is not very favorable 

 to such an hypothesis. The pectines are separated at their 

 outer ends from the body wall. The inpushings for the lung- 

 books are much deeper, and the cavity, which extends forwards 

 from the opening, is divided up by lamellae which grow down 

 from its upper end (PI. XVII, fig. 47). It is in close relation 

 to a space in the mesoblast which contains blood-corpuscles. 



The cephalic segment (PI. XVII, fig. 45) is now rapidly 

 approximating to its final shape. The cerebral ganglion, which 

 is seen from the surface as a four-lobed white mass (fig. 45, ce.), 

 has now lost all connection with the epiblast. The invagina- 

 tion remains, but its sides no longer give rise to nerve-tissue 

 (PI. XVII, figs. 48 and 49). The thickening for the central 

 eye (figs. 48 and 49, rin.) is more largely developed, and 

 pigment is deposited in the ends of the cells furthest from 

 the invagination. The eye is plainly visible as a double 

 black spot on the surface. The upper edge of the invagina- 

 tion is growing down to close its orifice. The hypodermis 

 lying immediately above it is clearly marked off from the 

 rest as the vitreous layer (fig. 49, vit.). A considerable space 

 still separates the retina from the vitreous layer. 



The lateral eyes now appear for the first time as black spots on 

 what Lankester terms the " optic area,''^ i. e. the front margin 

 of the head (PI. XVII, fig. 45, oc). Their development, as 

 Parker^ has shown, is strikingly difi'erent from that of the 

 central eyes. Each eye, and in this species there are at first 

 three, is formed (fig. 50) by a slightly cup-shaped thickening 

 of the hypodermis. The nuclei of this thickened portion 

 become larger, and pigment soon begins to be deposited at the 



* Loc. cit. 



