The Development of Amphioxus

Text Book of Biology, Part 1: Vertebrata by H. G. Wells, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. The Development of Amphioxus

The Development of Amphioxus

Section 12. The development of amphioxus, studied completely, is at once one of the most alluring and difficult tasks in the way of the zoologist; but certain of its earlier and most obvious fasts may very conveniently be taken into consideration now.), which divides the ovum into two parts. Another groove then cuts at right angles to this subdividing the two into four (). Another groove, at right angles to both the former, follows, making the four eight (). And so subdivision goes on. The whole process is called segmentation or cleavage., and of which an internal view, rather truer to the facts of the case as regards shape, is given as . The central cavity is the segmentation cavity (s.c.).). In this process a portion of the blastosphere wall is the tucked into the rest, as indicated by the arrow, so that a two-layered sack is formed. The space ar. is the archenteron, the primordial intestine, and its mouth is called, the blastopore (bp.). The outer layer of this double-walled sac is called the epiblast. For the present we will give the inner lining no special term. The young amphioxus has, at this stage, which is called the gastrula stage, a curious parallelism with such a lowly form as the Hydra of our ditches. This latter creature, like the gastrula, consists essentially of two layers of cells, an outer protective and sensory layer, and an inner digestive one; it has a primordial intestine, or archenteron, and its mouth is sometimes regarded as being a blastopore. All animals that have little yolk, and start early in life for themselves, pass through a gastrula stage, substantially the same as this of amphioxus. is a dorsal view of the gastrula at a somewhat later stage, and here indications of distinctly vertebrate relationships already appear. . is a cross-section, its position, being shown by cross-lines in 7 i. and 6. Note first that the epiblast along the mid-dorsal line is sinking in to form what is called the neural plate (n.p.), and simultaneously on either side of it rise the neural folds (n.f.). Now, at , a slightly later stage is represented, and at 9 i. the inturned part is separated from the general external epiblast as the spinal cord. The remainder of the epiblast constitutes the epidermis., along the dorsal side of the archenteron a thickening of its wall appears, and is gradually pinched off from it to form a cellular rod, lying along under the nervous axis and above the intestine. This is the notochord (compare ).. In  these buds have become hollow vesicles, growing out from it, the coelomic pouches. They are further developed in ; and in , which is a diagrammatic figure, they are indicated by dotted lines. They finally appear to (? entirely) obliterate the segmentation cavity-- they certainly do so throughout the body; and their cavities are in time cut off from the mesenteron, by the gradual constriction of their openings. In this way the coelom (body cavity) arises as a series of hollow "archenteric" outgrowths, and ms. becomes the alimentary canal. mt.c., the metapleural canals, probably arise subsequently to, and independently of, the general coelomic space, by a splitting in the body-wall substance. shows all the essential points of the structure of amphioxus. Epiblast is indicated by a line of dashes, mesoblast by dots, and hypoblast, dark or black. The true mouth is formed late by a tucking-in of epiblast, the stomodaeum (s.d.), which meets and fuses with the hypoblast, and is then perforated. The position of this mouth is at the velum. The formation of the atrium has been described. The metapleural folds run forward in front of the velum, as the epipleurs (ep. in ), and form an oral hood (b.c.), around which the tentacles appear, and which is evidently not equivalent to the vertebrate mouth cavity, but in front of and outside it. The anus is formed by a tucking in, the proctodaeum, similar to the stomodaeum.

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