46 



POLYPIFEUA. 



the two poles of the balloon becoming ap- 

 proximated, the whole embryo becomes some- 

 what disc-shaped, or the four vessels that 

 communicate with the stomach (if vessels they 

 really are), by moderately contracting, form as 

 many depressions dividing the embryo into 

 four lobes (fig. 50. 5, 6.), or by a more forcible 

 contraction give it the appearance of a Greek 

 cross, and all these changes of form may take 

 place in a few seconds. 



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 (Jig. 

 50.7) preparatory to their final establishment 

 as young Tubulariae (8, 9). 



Fig. 51. 



A. series showing the development of Tubularia coro- 

 natafrom ova. 



1. A ramification or bud of the ovary. The com- 

 mon cavity continued into it as a cul-de-sac, beyond 

 which is the ovum. 



2. The ovum becomes much enlarged, and sur- 

 rounding the cul-de-sac. 



3. The cul-de-sac turned aside by gentle com- 

 pression. Indentations on the ovuni indicating the 

 formation of tentacles. 



4. An elevation (b) in the centre of the tentacles 

 become perceptible, which afterwards forms the 

 proboscis-like part of the animal. 



5. The same compressed between two plates of 

 glass. 



6. The embryo after its escape from the ovisac, 

 having as yet but one row of tentacles. 



7. The young animal become fixed. The short 

 tentacles beginning to project at the anterior pro- 

 longation or proboscis. 



Third mode of propagation, by simple ova. 

 This mode of reproduction approximates the 

 nearest to what occurs in the higher animals. 



Cells arc 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 (Jig. 51. 

 4, b) is then perceived in the centre of the ten- 

 tacular zone, which becomes the proper 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 (Jig. 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 

 (Jig. 51.7). 



Prof. Van Beneden likewise witnessed the 

 same mode of propagation in Syncoryna pu- 

 silla. 



Fourth mode of propagation, by ova unth a 

 multiple vitellus. The fourth mode of repro- 

 duction observed by Professor Van Benedeu 

 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 similar. A bud is formed 

 from the surface of the parent zoophyte, in 



