GANOIDEI. 



95 



auditoi-y vesicles are now deep pits of the uervoiis layer of the epiblast, 

 the openincrs 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, x). 



In an embryo of the seventh 

 day after impregnation, the 

 features of the preceding stage 

 become generally more pro- 

 nounced. 



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. 



The optic vesicles are now 

 provided with a lumen (fig. 64), 

 and have approached close to the 



epidermis. Adjoining them a thickening [l) 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. 61, is bent inwards towards 

 the middle line, has now become slightly convoluted, and forms the rudi- 

 ment 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. (So. 



Both the head and tail are 

 quite free from the yolk ; and 

 the embryo presents a general 

 resemblance to that of a Tele- 

 ostean. 



On the ventral surface of 

 the front of the head there is 

 a disc (figs. 65, 66, sd), which 

 is beset with a number of 

 processes, formed as thicken- 

 ings of the epiblast. As shewn 

 by Agassiz, these eventually 

 become short suctorial pa- 

 pill8e\ Immediately behind 

 this disc is placed a narrow 



depression which forms the Fig. 64. Section through the front 



rudiment of the mouth. vkrt of the head of a Lepidosteus embryo 



rT\^ M' . -J. " on the SEVENTH DAY AFTER IMPREGNATION. 



The olfactory pits are now , ,. ^ . x ^x. *v, i i, 



, , , 111 al. alimentary tract; fo. tnalamencepna- 



developed, and are placed near j^j^. j ^^^g ^f gyg. ^^^^^ optic vesicle. The 



the front of the head. mesoblast is not represented. 



1 These papillae are very probably sensitive structures ; but I have not yet investi- 

 gated their histological characters. 



