3015. The blood-vessels of the uterus and placenta do not open into or communicate with each other.
3016. The fœtal water corresponds to the albumen or the white of eggs, not to the vitellus. This becomes also during incubation consumed in the formation of the body of the chick, and not the vitelline mass, which is first of all destined for the intestine.
3017. Towards the termination of pregnancy, when the fœtus is endowed with muscular motion, the fœtal water is also absorbed. Nutrition is therefore in the commencement an absorption carried on by the integument, and lastly by the intestine.
2. Respiration.
3018. The respiratory organ of the fœtus is the chorion, and in particular the placenta. Its tissue is like that of the branchiæ or spleen.
3019. The arterial blood transmitted through the umbilical vein is conducted through the foramen ovale into the left ventricle of the heart, and from thence directly to the principal organ of the fœtus, the brain and spinal cord. From hence it returns, venous in quality, to the right side of the heart, and from thence through the ductus arteriosus into the inferior or descending aorta, from which, pursuing its course through the umbilical arteries, it again reaches the placenta, in which it undergoes a renewed oxydation.
3020. If therefore the umbilical cord be compressed, the fœtus dies suddenly, as happens indeed accidentally in certain cases, which completely resemble those of death produced by suffocation. The chick when in the egg dies, if the egg-shell be coated over with varnish, or if the egg be submitted to the noxious influence of gases which are devoid of oxygen.
3021. The respiration effected through means of the placenta, admits also of being proved by the sudden change which occurs in the circulation after birth. No arterial blood being then any longer brought to the heart through the umbilical vein, the left heart is no longer stimulated, and the foramen ovale collapses and is closed. Thus, all the blood enters the right side of the heart, and finding no thoroughfare into the ductus arteriosus, it is driven forcibly into the lungs, which now expand, and thereby leave an empty space between the pulmonary vessels, into which the air rushes in.
3022. The first act of respiration is therefore the result of the lungs being injected with venous blood, and thus of necessity ensues. When, on the contrary, cases arise causing pulmonary suffocation, the blood then regurgitates to the umbilical vessels, in order to reach the original branchia or placenta.
3023. Another process of respiration is found to take place in the vessels of the allantois. Its fluid becomes oxydized and penetrates through the urinary cyst into what have been called primordial kidneys. This kind of respiration through the anus is persistent in many Worms and larvæ of Insects.