G04 ANATOMY OF VERTEBRATES. 



be formed by small folds, rising laterally, and overlapping the 

 fore-part of the second enlargement, ib. O, which has expanded to 

 greater breadth. The olfactory cavities appear as small cutaneous 

 depressions or follicles, ib. r. 



The two myelonal columns, expanding between the ear-sacs, 

 and receding so as to show the notochord beneath, bend npward 

 and inward, and unite, to be continued into the back part of 

 the optic lobes, thus commencing the cerebellar bridge, ib. C, 

 across the epencephalic ventricle. The encephalic vacuities have 

 begun to be filled by the granular basis of the cerebral fibre or 

 substance. The intestinal groove begins to be converted into a 

 canal at its two ends, which are closed : beneath the anterior end, 

 and behind the heart, progressively accumulates the cellular basis 

 of the liver. The free caudal end of the embryo grows rapidly ; 

 muscular heavings of the body occur before the heart beats, and 

 pulsation begins before the cavity is visible in the cell-mass. 

 The heart appears first as a cylinder of cells, changing in its 

 movements from a straight to a bent fissure, fig. 423, h, and pro- 

 pelling only colourless plasma ; a canal is next seen, along which 

 the blood-particles traverse the cylinder, from the yolk below to 

 the head above : these blood-particles are spherical or polyhedral, 

 colourless and homogeneous, and are more minute than the 

 germinal cells. The cardiac cylinder is next divided by a con- 

 striction into an auricular and a ventricular compartment. The 

 blood, in which the discs soon acquire increased size and a pale 

 red colour, 1 is propelled from the ventricle by channels encom- 

 passing the fore part of the alimentary canal into a dorsal trunk, 

 which, after a short course, bends down, and returns as a vein to 

 the vitellus, over which the blood at first courses in undeterminate 

 streams, but which converge to enter the auricular division of 

 the heart. As the abdominal cavity, intestine, and body elongate, 

 a succession of such vertical loops is formed, receding from the 

 first, with corresponding elongation of the aorta and postcaval, or 

 entero-vitelline, vein. The aorta soon sends off pairs of trans- 

 verse loops, corresponding with the vertebral segments, the 

 returning channels of which open into or constitute the cardinal 

 vein. The embryo now encompasses the yolk, as in fig. 422, g. 



In the eye the crystalline, developed from the epithelial layer 

 uniting the two ends of the bent ophthalmic vesicles, sinks deeper 



1 Lerebouillet observed in embryo-fishes raised in tanks from artificial impregna- 

 tion, that the blood-particles were later in formation, and more scanty than in the 

 embryos derived from the free streams : a remark worthy the attention of the breeder 

 of fish, cccxix. 580. 



