. A text-book of animal physiology, with introductory chapters on general biology and a full treatment of reproduction ... Physiology, Comparative. THE DEVELOPMENT OP THE EMBRYO ITSELF. 99 presents a curve to the right which represents the venous part and one to the left, answering to the arterial. The rudiments of the auricles also are to be seen. The arterial system is represented at this stage by the ex- panded portion of the heart known as the bulbus arteriosus, and two extensions from it, the aortae, which uniting above the alimentary canal, form a single posterior or dorsal aorta. From t
. A text-book of animal physiology, with introductory chapters on general biology and a full treatment of reproduction ... Physiology, Comparative. THE DEVELOPMENT OP THE EMBRYO ITSELF. 99 presents a curve to the right which represents the venous part and one to the left, answering to the arterial. The rudiments of the auricles also are to be seen. The arterial system is represented at this stage by the ex- panded portion of the heart known as the bulbus arteriosus, and two extensions from it, the aortae, which uniting above the alimentary canal, form a single posterior or dorsal aorta. From these great arterial ves- sels the lesser ones arise, and by sub- division constitute that great mesh- work represented diagrammatically in Figs. 112,113, from which the course of the circulation may be gathered. The beating of the heart commences be- fore the corpuscles have become nu- merous, and while the tubular system, through which the blood is to be driven, is still very incomplete. The events of the third day are of the nature of the extension of parts already marked out rather than the formation of entirely new ones. The following are the principal changes: The bending of the head-end down- ward (cranial flexure); the turning of the embryo so that it lies on its left side; the completion of the vitelline circulation; the increase in the curva- ture of the heart and its complexity of structure by divisions; the appear- ance of additional aortic arches and of the cardinal veins; the formation of four visceral clefts and five vis- ceral arches; a series of progressive changes in the organs of the special senses, such as the formation of the lens of the eye and a secondary optic vesicle; the closing in of the optic vesicle; and the formation of the na- sal pits. In the region of the future brain, the vesicles of the cerebral hemispheres become distinct; the hind-brain separates into cerebellum and medulla oblongata; the nerves, both cra-. Fio. 113.—Dia^am of the embry- on
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