Observations are wanting relative to the manner in which the free embryo is con verted into the fixed Tubularia ; for although Professor Van Beneden observed the latter at a very early period after they had become at tached, he was unable to witness the changes that they undergo at the moment of becoming attached to some foreign body, and therefore gives a hypothetical outline of the forms through which he supposes them to pass (fig. 50.7) preparatory to their final establishment a S young Tubularim (8, 9).
Third mode of propagation, by .simple ova. — This mode of reproduction approximates the nearest to what occurs in the higher animals.
Cells are observed to become organised in the middle of a vesicle in the same manner as the vitelline cells, and to become converted into an embryo. In this case the vitelline cells be come aggregated and modified, so as to give rise to a new individual, which is isolated from the commencement of its existence. The point of departure for the formation of the embryo is the same as in the preceding mode of development, and the reproductive vesicle has at first precisely the same structure as in the last case, but instead of preserving its transparency, this vesicle soon exhibits nume rous cells, which render it more and more opaque, and give it more the appearance of a vitellus. In this case moreover there is a great difference in the relations which the red pedicle (fig. 49, b) bears to the embryo. In the preceding mode of development this pe dicle constitutes an integrant part of the newly formed being, forming, in fact, its stomach, but in the oviparous mode there is no organic connection between the one and the other, the vitellus being formed between the pedicle and the integument of the offset, and on press ing the latter between two plates of glass these structures readily separate without any laceration.
As the vitellus increases in size it becomes impacted between the integument and the pe dicle, and its augmentation of size still in creasing, the upper part of the pedicle becomes covered with it as with a hood, and at last almost entirely enveloped by it. At this period the margins of the vitellus become in dented on that side nearest the pedicle, and the tubercles between the indentations soon show themselves to be the rudiments of ten tacula. The tentacula become more and more elongated, the embryo separates itself slightly from the pedicle, and a protuberance (fig. 51. 4, b) is then perceived in the centre of the ten tacular zone, which becomes the prof)er body of the polype, or rather forms the walls of its stomachal cavity.
The walls of the bud, which has hitherto contained the embryo, now become ruptured, and it gains its liberty (fig. .51. 6). In this condition it almost exactly resembles a young Hydra in its contracted state, and in fact both its body and its tentacula seem to have the same anatomical structure as those of that simply organised polype. Having attained to this condition its development proceeds ra pidly, and it soon begins to assume the specific form of the Tubularia from which it sprung (fig. 51. 7).
Prof. Van Beneden likewise witnessed the same mode of propagation in Syneoryna pu silla.
Fourth mode of propagation, by OVIT with a multiple vitellus.— The fourth mode of repro duction observed by Professor Van Beneden to occur among the tubular polypes is ex tremely curious. In this form, as in that last described, the young individuals are deve loped from ova, and the first steps of the pro cess are precisely shnilar. A bud is formed from the surface of the parent zoophyte, in the interior of which may be observed a ve sicle that soon becomes organised into nu merous cells, which constitute the vitelline mass exactly as in the last case. But, arrived at this point, the vitelline mass becomes tu berculated, assuming the appearance of a rasp berry, and, instead of a single vitellus, it is found to be an agglomeration of several, each of which contains in its interior a Purkingean vesicle from which a young individual is pro duced, which is of a totally different form from its parent and covered with cilia, by the aid of which it swims freely about in search of a locality where to fix itself. This form of re production will, however, be more particularly noticed in describing the Sertularian Polypes.
Fifth mode, hy free gemmcdion and ova com bined. — This last form of the reproductive process is merely a combination of two of the preceding, propagation being effected by the development of a free gemma, in the interior of which there is formed a divided vitellus. In this case a free embryo becomes organised, and takes the form of a young Medusa, accord ing to the second mode described above, in the interior of which is contained an ovum with a multiple vitellus, from which numerous ciliated embryos are produced, as in the Ser tularia genieulata hereafter to be noticed.