Fig. 60. Section through an embryo of Lepidosteus on the fifth day after impregnation.
MC. medullary cord; Ep. epiblast; Me. mesoblast; hy. hypoblast; Ch. notochord.
In the region of the tail, the axial part of the hypoblast, the notochord, and the neural cord fuse together, the fused part so formed is the homologue of the neurenteric canal of other types. Quite at the hinder end of the embryo the mesoblastic plates cease to be separable from the axial structures between them.
In a somewhat later stage the embryo is considerably more elongated, embracing half the circumference of the ovum. The brain is divided into three distinct vesicles.
Anteriorly the neural cord has now become separated from the epidermis. The whole of the thickened nervous layer of the epiblast appears to remain united with the cerebrospinal cord, so that the latter organ is covered dorsally by the epidermic layer of the epiblast only. The nervous layer soon however grows in again from the two sides.
Where the neural cord is separated from the epidermis, it is already provided with a well-developed lumen. Posteriorly it remains in its earlier condition.
In the region of the hind-brain traces of the auditory vesicles are present in the form of slightly involuted thickenings of the nervous layer of the epidermis.
Fig. 61. Embryo of Lepidosteus on the sixth day after impregnation.
op. optic vesicles; br.c. branchial clefts (?); s.d. segmental duct.
N.B. The branchial clefts and segmental duct are somewhat too prominent.