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.
In an embryo of the seventh day after impregnation, the features of the preceding stage become generally more pronounced.
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 rudiment of a pronephros (head-kidney).
Fig. 64. Section through the front part of the head of a Lepidosteus embryo on the seventh day after impregnation.
al. alimentary tract; fb. thalamencephalon; l. lens of eye; op.v. optic vesicle. The mesoblast is not represented.
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 beset with a number of processes, formed as thickenings of the epiblast. As shewn by Agassiz, these eventually become short suctorial papillæ[40]. Immediately behind this disc is placed a narrow depression which forms the rudiment of the mouth.