The Development of Alcyonium digitatum, with some notes on the Early Colony Formation.

Abstract

The fertilised eggs segment in various ways, but typical morulæ always result. When the sixteen cell stage again divides to produce the thirty-two celled embryo, delamination occurs, and from now onwards the larva is two-layered. The morula at the twentieth hour begins to undergo a series of contortions which last from the first to the third day. This solid contorted stage is here termed the pre-planula, as it passes on into the hollow planula stage. The pear-shaped planula, while swimming, exhibits characteristic “planarian-like” movements, and on the fourth free-swimming day (the seventh day of development) it settles down by the broad anterior and aboral pole. The settled larva soon flattens, assuming a mound-like shape, and ou the second day of fixation eight mesenteries grow out simultaneously into the coelenteron from the base of the lateral wall. The cœlenteron is identical with the hollow entrai space in the endoderm of the planula. The mesenteries, arranged in four distinct pairs, grow simultaneously and rapidly along the lateral walls and attached base, and soon nearly meet on the basal and oral surfaces of the polyp. Many round cells now appear at the base of the columnar ectoderm. Mesoglœa is at this time secreted by the endoderm, and flows round the above-mentioned round ectoderm cells, cutting them off either singly or in groups. These isolated ectoderm cells produce either nematocysts or spicules, the spicules appearing soon after the mesenteries. Early on the third day eight simple hollow tentacles grow out, alternating with the mesenteries, and encircling the oral surface. Later on the third day the stomodæüm and moufli arise by the appearance of a rapidly deepening invagination of the oral surface, in the centre of the circle of tentacles. Yolky detritus is still present in the coelenteron. On the fourth day the base of this invagination degenerates, and so the coelenteron communicates with the exterior. While the mouth invagination is still in process of formation, the endodermic portion of the mesenteric filaments arises on the six ventral mesenteries by a proliferation of cells on the upper part of the free edge. On the fifth day the ventral mesenteric filaments are completed by strap-like ectodermic downgrowths from the stomodæum over the endodermic thickening on each mesentery. On the sixth day the dorsal mesenteric filaments arise. The dorsal and ventral mesenteric filaments appear homogeneous in origin, though of diverse function. On the seventh day the eight filaments reach about half way down the free edges of their respective mesenteries. Further development consists of elaboration of the organs already present. At the end of the third week the first bud is formed, and the solitary polyp becomes a young colony by stolonal gemmation. The young colonies were successfully fed in the laboratory on larvæ and adult individuals from colonies of Lepto-clinum and Botryllus.