In a good many cases, as first pointed out by Blomfield[26], the whole of each spermospore does not become converted into spermatozoa, but part, either with or without a segment of the original nucleus, remains passive, and carrying as it does the off-budded spermoblasts may be called the 'sperm-blastophor.' This passive portion of protoplasm is not employed in the regeneration of the spermoblast. This very singular phenomenon has been observed in Elasmobranchii, the Frog, the Earthworm, Helix, etc.[27], and probably has a much wider extension. In Elasmobranchii (Semper) the passive portions of protoplasm are nucleated, and are placed on the outer side of the columnar spermospores which line the testicular ampullæ; they are not distinctly differentiated till the nuclei, segmented from the nucleus of the primitive spermospore to form the heads of the spermatozoa, have become fairly numerous. In the Frog the passive blastophor also occurs as a nucleated mass of protoplasm on the outer side of the spermospore. In the Earthworm the blastophor forms a central non-nucleated portion of the spermospore; and the whole periphery of each spermospore becomes converted into spermoblasts. [26] _Quart. Journ. of Micro. Science_, Vol. XX. 1880. [27] Blomfield, _loc. cit._, p. 83, states that he has observed this fact in Lumbricus, Tubifer, Hirudo, Helix, Arion, Paludina, Rana, Salamandra, and Mus. It has been already stated in the introduction that the male and female generative products are homodynamous, but the consideration of the development of the products in the two sexes shews that a single spermatozoon is not equivalent to an ovum, but rather _that the whole of the spermatozoa derived from a spermospore are together equivalent to one ovum_. CHAPTER II. THE MATURATION AND IMPREGNATION OF THE OVUM. _Maturation of the ovum and formation of the polar bodies._ In the preceding chapter the changes in the ovum were described nearly up to the period when it became ripe, and ready to be impregnated. Preparatory to the act of impregnation there take place however a series of remarkable changes, which more especially concern the germinal vesicle. The attention of a large number of investigators has recently been directed to these changes as well as to the phenomena of impregnation. The results of their investigations will be described in the present chapter; but for an historical account of these investigations, as well as for a determination of the delicate questions of priority, the reader is referred to Fol's memoir (No. 87), and to a paper by the author (No. 81). [FIG. 22. RIPE OVUM OF ASTERIAS GLACIALIS ENVELOPED IN A MUCILAGINOUS ENVELOPE, AND CONTAINING AN ECCENTRIC GERMINAL VESICLE AND GERMINAL SPOT (copied from Fol).]