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The males give out masses of sperm, and the females discharge ova in such quantity that many of them stick to the fibrils about their mouths. Both kinds of cells pass first into the mantle-cavity after the opening of the gonads, proceed through the gill-clefts into the branchial gut, and are discharged from this through the mouth. The ova are simply round cells.
They quite plainly exist to supply fine blood-vessels for breathing at the gill-clefts, and are never used, for the embryo does not breathe, except through the mother. They are a most instructive reminder of the Devonian fish which quitted its element and became the ancestor of all the birds and mammals of a later age.
At first there are very few of these branchial clefts; but there are soon a number of them first in one, then in two, rows. The foremost gill-cleft is the oldest. In the end we have a sort of lattice work of fine gill-clefts, supported on a number of stiff branchial rods; these are connected in pairs by transverse rods.
This transitory condition resembles that in which we find the nose permanently in the Dipneusts and Amphibia. A cone-shaped structure, which grows from below towards the lower ends of the two nasal processes and joins with them, plays an important part in the conversion of the open nasal groove into the closed canal. Below the mouth-pit are the gill-arches, which are separated by the gill-clefts.
When the ova of the female and the sperm of the male are ripe, they fall into the atrium, pass through the gill-clefts into the fore-gut, and are ejected through the mouth. Transverse section of the lancelet, in the fore half. Above the sexual glands, at the dorsal angle of the atrium, we find the kidneys. Transverse section through the middle of the Amphioxus.
The third branchial arch is only cartilaginous at the foremost part, and here the body of the hyoid bone and its larger horn are formed at each side by the junction of its two halves. They have been lost long ago. Moreover, the four gill-clefts of the human embryo are only interesting as rudimentary organs, and they soon close up and disappear.
And Rosenberg found it. In the same way the “gill-clefts” of the fish-like ancestors have long since been discovered in the embryo of the higher vertebrates and of man.
Man's ancestors are round coenobia or colonies of Protozoa; they consist of a close association of many homogeneous cells, and thus are individuals of the second order. Their body consists merely of a primitive gut, the wall of which is made up of the two primary germinal layers. The unsegmented chorda develops between the dorsal medullary tube and the ventral gut-tube. Head-gut with gill-clefts.
But the EXTERNAL, superficial branchial skeleton that supports the gill-crate in the Cyclostoma is replaced in the Gnathostomes by an INTERNAL branchial skeleton. It consists of a number of successive cartilaginous arches, which lie in the wall of the gullet between the gill-clefts, and run round the gullet from both sides.
Transverse section of an Amphioxus-larva, with five gill-clefts, through the middle of the body. The small particles that the Amphioxus takes in with the water infusoria, diatoms, particles of decomposed plants and animals, etc. pass from the gill-crate into the digestive part of the canal, and are used up as food.
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