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The presence of this hormone might cause the follicular cells to behave as though the ovum was still present in the follicle, so that they would persist and not die and be absorbed. But this leaves the question, what is lutein and why is it secreted? Lutein is a colouring matter sometimes found in blood-clots, and probably derived from haemoglobin.
And the resemblance becomes closer when we bear in mind that haemoglobin sometimes lies as a stationary deposit in certain tissues, like the tongue muscles of certain mollusks, or the nerve cord of Aphrodite and Nemerteans.
It transpired that the red corpuscles are composed chiefly of a substance which Kuhne first isolated in crystalline form in 1865, and which was named haemoglobin a substance which has a marvellous affinity for oxygen, seizing on it eagerly at the lungs vet giving it up with equal readiness when coursing among the remote cells of the body.
In some of the Nemertina the blood is already coloured, and the red colouring matter is real haemoglobin, connected with elliptical discoid cells, as in the Vertebrates. The further evolution of this rudimentary vascular system can be gathered from the class of the Annelids in which we find it at various stages of development.
Nor do the services of the Philozoon end here; for during sunlight it is constantly evolving nascent oxygen directly into the surrounding animal protoplasm, and thus we have actually foreign chlorophyl performing the respiratory function of native haemoglobin!
In them we find blood-vessels for the first time, distributing real blood through the body. The blood is red, and the red colouring-matter is haemoglobin, connected with elliptic discoid blood-cells, as in the Vertebrates.
Precisely the same process is going on continuously and unobtrusively within the human body under normal bodily conditions of temperature and pressure, when the oxy-haemoglobin of the arterial blood changes over into the haemoglobin of the venous blood.
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