But even here it is the nuclei of the seminal cells, which serve for the deveTopernent of the spermatozoa. The whole difference consists in this, that the nuclei are metamorphosed altogether into the fructifying elements of the semen, whilst otherwise they produce the spermatozoa in the interior, they themselves getting dissolved when the latter are about to be liberated. The external cellular mem brane which encloses these miclei reinains, however, without any immediate participation in the formation of the spermatozoa. It gets destroyed in the course of the developement, in order to enable the nuclei, which in the mean time have been converted into sperma tozoa, to make their exit. This at least holds good in most of these cases, the Nernatoda only being an exception. The membrane of the cells belonging to the metatnorphosed nuclei, persists in the latter-named animals.
According to this we have a threefold mode of developement of the spermatozoa, viz.:— 1st. The cell membrane and nucleus of the formative vesicles convert themselves imine diately into the spermatozoon.
2d. The nucleus of the formative vesicles alone metamorphoses itself into the sperma tozoon.
3d. A new formation, which talces place in the interior of the nucleus (or immediately in the cell cavity), performs the functions of a spermatozoon.
On comparing the spermatozoa developed in these different ways, we cannot deny that they have a different stage of development in a morphological point of view. The sper matozoa resulting from endooenous forma tion are most highly developd; they are the produce of a perfectly new generative process, whilst the other forms of spermatozoa owe their origin to a persistency and further development of structures, which, first of all, were mere transitory elements, and were only of importance as the seat of that neoplastic process. Under such circumstances we may assume, then, that all these forms of sperma tozoa, according to the morphological rela tion in which they stand, are mere different stages of developetnent in one common con tinued series,—mere variations of one theme, in which the differences seen are not essen tial, but only of a relative import. Taking into consideration this unity, we cannot agree to the objection that may possibly be made to us, as if we had described the spermatozoa (which, essentially and in fact, are identical formations) to have been pro duced in different ways. The mutual rela tion of these differences is in. perfect unison
with the laws of organic architecture, which every where (when a common pldn is made the basis of a series of formations) exhibits the variety, of the concrete form principally through a variable developement and perfec tion of the ideal type.
It might not be without interest to re flect upon the important part which the nucleus plays in the formation of the sperma tozoa, since it is an element which is usually only important for the formation of cells, and does not participate in their subsequent me tamorphoses. This at least is the rule; a rule, however, by no means without excep tion. We already know that in many cases the nucleus is hnportant for the developement of certain parts ; we know that the nucleus in many glandular cells of the insects gra dually assumes a remarkable ramified shape* ; and that it even converts itself in other cases into peculiar fibrous formations — into the so-called nuclear fibres (Kernfasern).± Still more remarkable is the metamorphosis of the nucleus in the developement of the so called prickle or nettle organs — those in teresting microscopical formations, which are so frequently imbedded in the skin of the lower animals (e.g. Polyps and Medusw), and which present so great a similarity to certain forms of the seminal fibres, that they were even taken for such by one of us on their first discovery. K011iker'st observa tions, as well as our own more recent ones, instituted upon Hydra, convince us beyond doubt that it is the nuclei of cells which gradually metamorphose themselves into the capsules of the prickles, and which ultimately become free through the dissolu tion of the cell membrane surrounding them. The same genetic process therefore takes place here in every essential point of view, as that by which the formation of the sperma tozoa in the Chilopoda is effected. But the developement of the prickles is never limited to this metamorphosis of the nucleus. There is formed at the same time in its interior a peculiar linear or fibrous part, which how ever constantly enters into combination with the persistent external vesicle of the nuclei or with the capsule. Thus we may see that the formation of the prickles is closely con nected with that scheme which we have laid down as a formula for the developement of the spermatozoa. It occupies the medium be tween the second and third mode of de velopement of the _spermatozoa established by us.