The differentiation and the confluence of the cell constituents of the primitive streak have led to the formation of a pair of albuminous cords along the sides of the median furrow, forming the _myelon_ proper. The cells exterior to and above them are converted into muscle and fibrous septa; and beneath the column is a jelly-filled cylinder, with a transversely striated sheath, pointed at both ends, forming the _notocord_. Its anterior point passes a little in advance of the acoustic vesicle. Beneath the notocord and surrounding the blastema is stretched the vegetative, or mucous, layer of cells, in contact with the yolk. Both the head and the tail of the now cylindrical embryo are liberated from the surface of the yolk. A fold of the blastema, reflected from the under part of the head, sinks like a pouch into the yolk, and soon includes the rudiment of the heart, like a bent cord, which begins to oscillate about the seventh day. From the midline of the inferior surface of the embryo, or its mucous layer, two longitudinal plates descend, diverging into the yolk-substance, and form the _primitive intestinal_ groove. The _ophthalmic vesicle_ elongates and curves outward until the two ends almost come into contact. Between these two ends and beneath the delicate tegumentary layer connecting them the _crystalline lens_ is formed. About the same time, the _otoliths_ appear in the acoustic vesicles, which have now acquired a cartilaginous case. The _cerebral lobes_ begin to be formed by a small fold, rising laterally and overlapping the forepart of the second enlargement, which has expanded to greater breadth. The _olfactory cavities_ appear as small cutaneous follicles. The two myelonal columns, expanding between the ear sacs and receding so as to show the notocord beneath, bend upward and inward, and unite, to be continued into the posterior of the _optic lobes_, thus commencing the cerebellar bridge across the epencephalic ventricle. The encephalic vacuities have begun to be filled by the granular basis of the cerebellar substance. The _intestinal groove_ begins to be converted into a canal at its two ends. Beneath the anterior end, and behind the heart, there gradually accumulates the cellular basis of the liver. The commencement of the development of the _organ of hearing_ is by a superficial depression of the cephalic blastema to meet the process from the encephalon, which forms the acoustic nerve. The lining of the depression becomes, on closure of the slit, the proper tunic of the labyrinth.