DEVELOPMENT OF TUBULARIA CORONATA. 91 



envelope, like a worm in search of a passage by which to get out ; and 

 at length it pushes its free extremity through the opening in front of 

 it, and elongates its body still more, until the two poles of the balloon 

 getting approximated, the whole embryo becomes somewhat disk- shaped, 

 or the four vessels that communicate with the stomach (if vessels they 

 really are), by moderately contracting, form as many depressions di- 

 viding the disk into four lobes (fig. 43, H, i), or by a more forcible con- 

 traction give it the appearance of a Greek cross ; and all these changes 

 of form may take place in a few seconds. 



(211.) Observations are wanting relative to the manner in which 

 the free acaleph gives origin to the fixed polyp ; for although Professor 

 Van Beneden observed the latter at a very early period after they had 

 become attached, he was unable to witness any further changes that 

 they undergo, and therefore gives a hypothetical outline of the forms 

 through which he supposes them to pass, preparatory to their final 

 establishment as young Tubulariae. 



(212.) Third mode of propagation,, by simple ova. This mode of repro- 

 duction approximates the nearest to what occurs in the higher animals. 

 Cells are observed in process of gradual organization in the middle of 

 a vesicle, in the same manner as the vitelline cells, which are converted 

 into an embryo. In this case the vitelline cells become 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 ; but instead of preserving its transparency, this vesicle 

 soon exhibits numerous cells, which render it more and more opake, 

 and give it the appearance of a vitellus. In this case, moreover, there 

 is a great difference in the relations which the red pedicle (fig. 44, A, B), 

 bears to the embryo. In the preceding mode of development this 

 pedicle constitutes an integrant part of the newly-formed being, form- 

 ing, in fact, its stomach ; but in the oviparous mode, there is no organic 

 connexion between the one and the other, the vitellus being formed 

 between the pedicle and the integument of the offset, and on pressing 

 the latter between two plates of glass, these structures readily separate 

 without any laceration. 



(213.) As the vitellus (fig. 44, B, a) increases in size, it becomes im- 

 pacted between the integument and the pedicle, and its augmentation 

 of bulk still increasing, the upper part of the pedicle becomes covered 

 with it as with a hood, and at last almost entirely enveloped by it (fig. 

 44, c, D, E). At this period the margins of the vitellus become indented 

 on that side nearest the pedicle, and the tubercles between the inden- 

 tations soon show themselves to be the rudiments of tentacula. The 

 tentacula become more and more elongated, the embryo separates itself 

 slightly from the pedicle, and a protuberance (fig. 44, p, G, 6) is then per- 



