In many cases Rotifers lay summer and winter eggs of a different character. The former are always provided with a thin membrane, and frequently undergo development within the oviduct. They are hatched in the autumn. The winter eggs are always provided with a thick shell. The summer eggs are of two kinds, viz. smaller eggs which become males, and larger, females. On the authority of Cohn (No. 232) they are believed to develop parthenogenetically. Males are not found in summer, and only seem to be produced from the summer eggs. Cohn's observations, especially on _Conochilus volvox_, are however not quite satisfactory. Huxley (No. 234) came to the conclusion that the winter eggs of Lacinularia developed without previous fertilization. The following are the more important results of Salensky's observations (No. 236) on _Brachionus urceolaris_. 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-oesophageal 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-oesophageal ganglion.]