Vietnam or Thailand ? Vote for the TOP Country of the Week !
Updated: May 15, 2025
The magnet that attracts iron filings, the powder that explodes, the steam that drives the locomotive, are living inorganics; they act by living force as much as the sensitive Mimosa does when it contracts its leaves at touch, or the venerable Amphioxus that buries itself in the sand of the sea, or man when he thinks.
We should expect some such correspondence from the fact already stated that the embryonic development of the individual is a brief recapitulation of the ancestral development of the species or larger group. The egg of the lowest vertebrate, amphioxus, shows these changes in a simple and apparently primitive form.
From the concordant observations of Kowalevsky at Naples and Hatschek at Messina, it follows, firstly, that the ovum-segmentation and gastrulation of the Amphioxus are of the simplest character.
This resemblance is so close in the main features that we have only to repeat what we have already said of the ontogenesis of the Amphioxus. In the thick fine-grained yelk we find a clear round germinal vesicle of about 1/750 of an inch in diameter, and this encloses a small embryonic spot or nucleolus.
He must have been at least ten or twelve years ahead of Yarrell; and he saw the whole significance of it, too he saw where it led to. As I understand it, he actually anticipated in his pamphlet Saint Hilaire's theory of the universal type, and supported the hypothesis by describing the notochord of the amphioxus as a cartilaginous vertebral column.
But this must not be understood in the sense that the existing Amphioxus, or the sharks or amphibia of to-day, can give us any idea of the external appearance of these remote stem-forms. Still less must it be thought that the Amphioxus or any actual shark, or any living species of amphibia, is a real ancestral form of the higher vertebrates and man.
When we compare this embryonic condition, the sandal-shaped foetus, with the developed lancelet, we may say that the amphioxus is, in a certain sense, a permanent sandal-embryo, or a permanent embryonic form of the Acrania; it never rises above a low grade of development which we have long since passed. The body is pointed at both ends, but much compressed at the sides. There is no trace of limbs.
It is also called the branchial cavity and the cloaca, because it receives the excrements and sexual products as well as the respiratory water. This is so like the gill-crate of the Amphioxus in its whole arrangement that the resemblance was pointed out by the English naturalist Goodsir, years ago, before anything was known of the relationship of the two animals.
In the middle line of its ventral side we find the hypobranchial groove. The ejection-opening of this peribranchial cavity corresponds to the branchial pore of the Amphioxus. In the adult Ascidia the branchial gut and the heart on its ventral side are almost the only organs that recall the original affinity with the vertebrates.
It then divides into two sections, a fore or branchial gut and a hind or hepatic gut, like the alimentary canal of the Balanoglossus, the Ascidia, and the Amphioxus. But the branchial gut, the one reminiscence of our fish-ancestors, is afterwards atrophied as such. The parts of it that remain are converted into entirely different structures. Transverse section of the head of a Petromyzon-larva.
Word Of The Day
Others Looking