92 MORPHOLOGY. 



vesicle or spot can lie found either in the entire mass or in any of the detached portions, unless 

 the nucleated cells just referred to (fig. 22) can be so regarded; so also the phenomenon of yolk- 

 cleavage, if present at all, is very obscure, but the detached mass may be easUy broken up into 

 cells filled with secondary cells. 



The ovum (for I have no hesitation in so designating the mass detached from the primitive 

 plasma, notwithstanding its anomalous character) lies in contact with the remainder of the 

 plasma, and while in this position becomes developed into an actiniform embryo, as has been already 

 noticed by Van Bcnedcn,^ Mummery,' myself,' and others. In the act of development, as shown 

 in figs. 11 — 16, which represent the corresponding process in Tubularia indivisa, it becomes first 

 (fig. 13) extended as a disc over the residual plasma. In this disc we can always recognise a 

 difierentiation between its peripheral and central portions. Next (fig. 14), from the circumference 

 of the disc short and thick processes radiate all round, and these soon elongate themselves into 

 tentacles (fig. 15) ; the disc at the same time gradually becomes more gibbous on the side turned 

 away from the axis of the gonophore, its interior has already become hollowed out into a digestive 

 cavity, and a mouth now makes its appearance in the centre of the opposite side, or that in 

 contact with the plasma. The embryo now retreats from the plasma, the, mouth is seen to be 

 elevated on a conical prominence (fig. 16 a, fig. 24//), while the side opposite to the mouth 

 becomes more and more prolonged with the general cavity of the embryo continued into it. The 

 extremity of this prolongation presents in Tubularia larynx and some other species the appearance 

 of delicate strise (probably fibres) radiating for a short distance from its central point (PI. XXI, 

 fig. 6) — a peculiar structure which might easily lead to the belief that an aperture was here present. 

 The appearance of an aperture, however, I believe to be entirely deceptive. In this state it escapes 

 from the gonophore, a circle of very short tentacles having first become developed round the mouth 

 in some species {T. indivisa, fig. 16) ; while in others (7! larynsc, fig. 24) the oral tentacles do not 

 make their appearance until after the escape of the embryo. After continuing free (PI. XX, fig. 4, 

 PI. XXI, fig. 6) for a period, the side opposite to the mouth becomes ultimately developed into a 

 cylindrical stem, which soon clothes itself with a perisarc and fixes the young Tubularia to some 

 neighbouring oliject (PI. XX, fig. 5, PI. XXI, fig. 7). After the escape of the embryo, or even 

 during its development within the gonophore, the remaius of the plasma may stiU throw off 

 portions (PI. XXIII, fig. 24/), which become developed, in a similar way, into free actiniform 

 embryos. To such embryos the name of actinulm may be given, in order to distinguish them 

 from \\\& planulm of other hydroids.* 



^ " Recherches sur rEmbryogenie des Tubulaires," p. 37, pi. 1, in ' Nouv. Mem. de I'Acad. Uoy. 

 de Bruxelles,' torn, xvii, 1814. 



^ " On the Development of Tubularia indivisa" 'Trans. Micr. See.,' 1853, p. 28. 



^ Allman, " On Tubularia indivisa," ' Ann. Nat. Hist.,' July, 1859. 



* Prof. H. J. Clark has given a detailed account of the development of the gonophore and 

 ovarian plasma in Tubularia (" Tubularia not Parthenogenous/' ' American Journal of Science and 

 Arts,' vol. xxxvii, Jan., 1864). I cannot, however, accept in all points his interpretation of the appear- 

 ances presented in the microscopic investigation of these parts. He regards as the true ova certain 

 very minute cells which are visible in the gonophore while yet in a rudimentary state, and which 

 would seem to be those described above as composing the very young tissue of the plasma. Notwith- 

 standing, however, a certain resemblance of these cells to ova, I cannot so regard them. They cannot 

 be followed through any of the changes which characterise the development of a true ovum ; they 



