This conclusion is, however, disputed in a later work by Foot ('96), who maintains that the yolk-nucleus in Allolobophora is not of nuclear but of " archoplasmic " origin, though no relation between it and an attraction-sphere is established.' She adds the very interesting discovery that the " polar rings " (cf. p. c so) are probably to be identified with the yolk-nucleus, or are at least derived from a similar substance.
Calkins's observations taken in connection with those of Balbiani, Van Bambeke, and other earlier workers give, however, strong evidence, as I believe, that the " yolk-nucleus " of Lumbricus is derived, if not from the nucleus, at any rate from a substance nearly related with chromatin, which is afterwards converted into cytoplasmic substance. It is certain, in this case, that the appearance of the yolk-nucleus is coincident with a rapid growth of cytoplasm ; but we cannot suppose that the latter grows entirely at the expense of the yolk-nucleus. More probably the yolk-nucleus supplies certain materials necessary to constructive metabolism, and it is not impossible that these may be ferments. We may perhaps interpret in the same manner the elimination of separate nuclear elements (i.e. not forming a definite yolk-nucleus) as described by Van Bambeke, Mertens, v. Erlanger, and many earlier writers.
The meaning of the yolk-nuclei of purely cytoplasmic origin is very obscure, and we have at present really no ground for assigning to them any particular function. It can only be said that their appearance coincides in time approximately with the period of greatest constructive activity in the cytoplasm, but there is no evidence of their direct participation in the yolk-formation, and we do not know whether they are active constructive physiological centres, or merely stores of reserve substances or degeneration-products.
2. Formation of the Spermatozoon Owing to the extreme minuteness of the spermatozoon, the changes involved in the differentiation of its various parts have always been, and in some respects still remain, among the most vexed of cytological questions. The earlier observations of Schweigger-Seidel, and La Valette St. George, already mentioned, established the fact that the spermatozoon is a cell ; but it required a long series of subsequent researches by many observers, foremost among them La Valette St. George himself, to make known the
general course of spermatogenesis. This is, briefly, as follows : From the primordial germ-cells arise cells known as which at a certain period pause in their divisions and undergo a considerable growth. Each spermatogonium is thus converted into a spermatocyte, which by two rapidly succeeding divisions gives rise to four spermatozoa, as The primary spermatocyte first divides to form two daughter-cells known as spermatocytes of the second order or sperm mother-cells. Each of these divides again— as a rule, without pausing, and without the reconstruction of the daughter-nuclei — to form two spermatids or sperm-cells. Each of the four spermatids is then directly transformed into a single spermatozoon, its nucleus becoming very small and compact, its cytoplasm giving rise to the tail and to certain other structures. The number of chromosomes entering into the nucleus of each spermatid and spermatozoon is always one-half that characteristic of the tissue-cells, and this reduction in number is in many cases effected during the two divisions of the primary spermatocyte. In some cases, however (e.g. in the salamander), the reduced number appears during the division of the spermatogonia and may even appear in the very early germ-cells (cf. p. 194). The reduction of the chromosomes, which is the most interesting and significant feature of the process, will be considered in the following chapter, and we are here only concerned with the transformation of the spermatid into the spermatozoon. All observers are now agreed that the nucleus of the spermatid is directly transformed into that of the spermatozoon, the chromatin becoming extremely compact and losing, as a rule, all trace of its reticular structure. It is generally agreed, further, that the envelope of the tail-substance is derived from the cytoplasm of the spermatid. Beyond this point opinion is still far from unanimous, though it is probable that the other structures — viz. the axial filament, the middle-piece, and the point — are likewise of cytoplasmic origin ; and it is certain that the middle-piece is in some cases derived from the attraction-sphere of the spermatid, and contains the centrosome.