322 ANATOMY OF THE DOMESTIC FOWL 



of aponeurotic septa probably accompany and support nervous 

 productions from the myelon columns. 



Two transverse constrictions begin to divide the cephalic en- 

 largements into three lobes, the second and the third of which ex- 

 pand into vesicles. An accumulation of cells at the side of the 

 middle expansion appears to add greatly to its breadth. This 

 forms the basis of the eyes. 



The differentiation and the confluence of the cell constituents 

 of the primitive streak have led to the formation of a pair of albu- 

 mmous 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 jeUy- 

 fiJled 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 sur- 

 rounding 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 hberated 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 rudi- 

 ment 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, diverg- 

 ing 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 crystal- 

 line 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 foUicles. 



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 epencephaUc 

 ventricle. The encephaUc 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 



