GANOIDEI. 



The optic vesicles are very prominent outgrowths of the brain, but are 

 still solid, though the anterior cerebral vesicle has a well-developed lumen. 

 The auditory vesicles are now deep pits of the nervous layer of the 

 epiblast, the openings of which are covered by the epidermic layer. They 

 are shewn for a slightly later 

 stage in fig. 63 (au.v.}. 



There is now present a sub- 

 notochordal rod, which develops 

 as in other types from a thick- 

 ening of the hypoblast (fig. 

 62, *). 



In an embryo of the 

 seventh day after impreg- 

 nation, the features of the 

 preceding stage become 

 generallymore pronounced. 



FIG. 63. SECTION THROUGH THE HEAD 

 OF A LEPIDOSTEUS EMBRYO ON THE SIXTH 

 DAY AFTER IMPREGNATION. 



au.v. auditory vesicle ; au.n. auditory 

 nerve ; ch. notochord ; hy. hypoblast. 



fb 



op 



The optic vesicles are now 

 provided with a lumen (fig. 64), and have approached close to the epidermis. 

 Adjoining them a thickening (/) of the nervous layer of the epidermis has 

 appeared, which will form the lens. 

 The cephalic extremity of the 

 segmental duct, which, as shewn 

 in fig. 6 1, is bent inwards towards 

 the middle line, has now become 

 slightly convoluted, and forms the 

 rudiment of a pronephros (head- 

 kidney). 



During the next few days 

 the folding off of the embryo 

 from the yolk commences, 

 and proceeds till the embryo 

 acquires the form represented 

 in fig. 65. 



Both the head and tail 

 are quite free from the yolk ; 

 and the embryo presents a 

 general resemblance to that 

 of a Teleostean. 



On the ventral surface of 

 the front of the head there is a disc (figs. 65, 66, sd), which is 



82 



FIG. 64. SECTION THROUGH THE FRONT 

 PART OF THE HEAD OF A LEPIDOSTEUS 

 EMBRYO ON THE SEVENTH DAY AFTER 

 IMPREGNATION. 



al. alimentary tract ; fb. thalamencepha- 

 lon; /. lens of eye; op.v. optic vesicle. The 

 mesoblast is not represented. 



