The ovum is attached by a short stalk to the hind end of the body of the female, in which position it undergoes its development. It will be convenient to treat separately the development of the female and male, and to commence with the former. The female ovum divides into two unequal spheres, of which the smaller in the subsequent stages segments more rapidly than the larger. The segmentation ends with the formation of an epibolic gastrula. The solid inner mass of cells derived from the larger sphere constitutes the hypoblast, and is more granular than the epiblast. The evolution of the embryo commences with the formation of a depression on the ventral surface, at the bottom of which the stomodæum is formed by an invagination. At the hinder part of the depression there rises up a rounded protuberance which eventually becomes the caudal appendage or foot. Immediately behind the mouth is formed an underlip.
On the sides of the ventral depression are two ridges which form the lateral boundaries of the trochal disc. They appear to unite with the under lip.
In a later stage the anterior part of the body becomes marked off from the posterior as a præoral lobe, and the hypoblast is at the same time confined to the posterior part. The supra-œsophageal ganglion is early formed as an epiblastic thickening on the dorsal side of the præoral lobe.
The first cilia to appear arise at the apex of the præoral lobe. At a later period the lateral ridges of the trochal disc meet dorsally and so enclose the præoral lobe. They then become coated by a ring of cilia, to which a second ring, completing the double ring of the adult, is added later.
Fig. 100. Embryo of Brachionus urceolaris shortly before it is hatched. (After Salensky.)
m. mouth; ms. masticatory apparatus; me. mesenteron; an. anus; ld. lateral gland; ov. ovary; t. tail, i.e. foot; tr. trochal disc; sg. supra-œsophageal ganglion.
In the trunk an indication of a division into two segments makes its appearance shortly after the development of the præoral lobe. Before this period the proctodæum is established as a shallow pit immediately behind the insertion of the foot. The latter structure soon becomes pointed and forked ([fig. 100], t).
The complete establishment of the alimentary canal occurs late. The stomodæum ([fig. 100]) gives rise to the mouth (m), œsophagus and masticatory apparatus (ms). The mesenteron is formed from the median part of the hypoblast; the lateral parts of which appear to give rise to the great lateral glandular structures (ld) which open into the stomach, and to the ovaries (?) (ov) etc. The proctodæum becomes the cloaca and anus (an). The origin of the mesoblast is not certainly known. The shell is formed before the larva is hatched—an occurrence which does not take place till the larva closely resembles the adult.