The decision of the question respecting the histological significance of the vesicles of de velopement is much more difficult. In many cases, especially when they are situated sepa rately or in small numbers in the interior of the spermatic cells, they have evidently the value of nuclei. Whether this however is always and every where the case, as lallikev supposes, we would not assert ; the less so because the appearance and the vesicular form of these structures do not by any means enable us to distinguish them properly from cells void of nuclei.
By the laws of analogy, we are, however, perhaps justified in forming a judgment on the nature of the respective elements even in such doubtful cases. We ourselves might perhaps even venture to pronounce that the vesicles of developement of the spermatozoa are in all cases nuclei. The unity in the mode of developement of the spermatozoa which would thus be established is certainly very attractive; but we dare not conceal it from ourselves that this inference from analogy is the less to be depended upon, since the genesis of the spermatozoa in the Decapoda furnishes us with a proof that the formation of these elements may also take place imme diately in the interior of cells, without the nuclei at all participating in it. We are confirmed in this opinion from the circum stance that in many Decapoda, for instance in Mysis, it is not the cell itself in which the spermatozoa are produced. The cylindrical staff, in the interior of which the spermatozoa are developed, is the produce of the metamor phosis of this cell ; a metamorphosis which here appears in its extreme form, but which in other cases is less striking, and may even be entirely wanting. And then it is the cell in its unchanged form which appears as the vesicle of developement of the spermatozoa.
Under such circumstances it might for the present be venturing too much to sever the mode of developement of the spermatozoa in the Decapoda, as a particular form, from the ordinary endogenous formation of these elements. We are not justified in so doing, until we have proved that in other animals mere nuclei exist as the mother cells of the spermatozoa. It is possible that such a proof may yet be established, indeed even probable, when we consider that there is also in other respects a difference in the formation of the seminal fibres between the Decapoda and other animals, inasmuch as the vesicles of developement in the former generally and almost constantly produce a greater number of spermatozoa, whilst in other animals they only produce a single fibre.
As a circumstance of subordinate import, which need not influence our judgment re specting the nature of the vesicles of de velopement, we may specify a difference in the histological characters of these parts, which certainly at first sight must appear very striking. We meet with them either in an independent free state, or separate, or en closed in a variable number within cells, which themselves not unfrequently hang to gether in groups, or are even situated in a cyst-like enclosure. But all these differ ences result solely from a different develope ment and application of all the plastic capa cities inherent in the cells. They are due to the occurrence of an endogenous multipli cation, and are readily explained by the in timate unity and connection which this method of developement presupposes.
The formative elements of the semen ap pear to us in their primitive forin as simple nucleated cells. But it is only rarely that they retain this original form. As a rule they present only, the starting point of a series of metamorphoses, which essentially are limited to a new formation of nuclei, or even of perfect nucleated cells in the interior of the primitive spermatic cell: a new form ation, which, however, not unfrequently oc casions the destruction of the mother cell.
It is not yet decided in all cases in what manner the formation of the daughter cells takes place, whether in the usual mode of endogenous cell formation, or by enclosure of portions of the contents. It seems, how ever, that the former mode of production is by far the more frequent one. Reichert has been the only one who has hitherto dis covered a formation of daughter cells round portions of the contents (like the forma tion oF cells in the minutely divided yolk) in the spermatic cells of the Nematoda. If such discovery should be confirmed — should it even have a greater extension — we may then further presume (as Kolliker already observes) that these two modes of develope ment are not essentially different from each other.