United States or Anguilla ? Vote for the TOP Country of the Week !
On these principles I assume that the oldest and simplest Turbellaria arose from Platodaria, and these directly from bilateral Gastraeads. The chief advances were the formation of gonads and nephridia, and of the rudimentary brain.
The problem of rejuvenation is the problem of recharging, or replacing all of the glands of internal secretion, at least the most important, the thyroid, the pituitary and the adrenals, as well as the gonads. Longevity is perhaps largely a matter of preventing, or postponing their wane.
This seems so very improbable that it suggests a doubt whether the same investigator was not mistaken with regard to the results of his experiments in transplanting gonads in Moths. Smith and Schuster conclude that the normal development of the thumb-pad depends on the presence of normal testes, but that there is no sufficient evidence that the effect is due to a hormone derived from the testis.
Under this is a soft medullary substance, which consists of entodermic cells with vacuoles. These gonads are not yet independent sexual glands, but sexually differentiated cell-groups in the medullary substance, or, in other words, parts of the gut-wall. Most of the Platodaria have not the muscular pharynx, which is very advanced in the Turbellaria and Trematoda.
Here, however, we are more especially concerned with the gonads and other reproductive organs. A great deal of evidence has now been obtained that the influence of the testes and ovaries on secondary sexual characters is due to a hormone formed in the gonads and passing in the blood in the course of the circulation to the organs and tissues which constitute those characters.
One of these is to graft ovaries or testes from one animal into another which possesses a certain somatic character, and then to see if the offspring produced from these gonads shows any trace of the character of the foreign soma in which it was nourished. Exper. He grafted the ovaries of two Black Leghorn pullets into two White pullets of the same breed, and vice versa.
Thus in the human species, it is possible for one type of sex glands to exist in the opposite type of body, as we saw it to be in cattle though it apparently could not occur unless compensated for in some way by the other secretions. This is a very great departure from birds, rats and guinea pigs, whose bodies change over their sex type when the gonads are transplanted. This is not true of man.
Besides the ante-pituitary, in the evolution of judgment, and the judgment faculty, due stress must be laid upon the influence of the internal secretion of the testes or ovaries, the product of the interstitial cells. With the advent of the gonads upon the scene, judgments become the centre of the play's plot undoubtedly. The intelligence of eunuchs and eunuchoids is in general low.
The occurrence of male somatic characters on one side or in some part of the body and female on the other, usually associated with the corresponding gonads, has been termed gynandromorphism, and has long been known in insects. Cases of this condition have been observed, though much more rarely, in Vertebrates.
No character whose development is dependent in greater or less degree on the stimulation of some substance derived from the gonads can have originated as a mutation, because the term mutation means a new character which develops in the soma as a result of the loss or gain of some factor or determinant in the chromosomes.
Word Of The Day