498 PEOF. ALLMAN ON TUE STKUCTUEE AND 



Metsclinikoff, in a sliort communication to the St. Petersburg 

 Academy *, describes the eggs o£ Alcyonella as formed in the 

 inner epithelial layer of the body-cavity, where they occur as 

 simple cells combined into a mass so as to form an ovary. From 

 this are detached the mature eggs with the germinal vesicle still 

 apparent. These float about for a time in the body-cavity, and 

 then enter into relation with a peculiar bud, which appears, in 

 the form of an ordinary Polyzoon bud, on the walls of the body- 

 cavity, into which it projects. He could not discover how the 

 egg becomes first attached to this bud ; but he has determined 

 that it ultimately becomes included^ within it, the bud enveloping 

 it in a duplicature which he compares to a decidua reflexa. In 

 the sort of brood-capsule thus formed the egg undergoes total 

 cleavage, and becomes changed into a heap of cells, which, after 

 enlarging, forms a central cavity surrounded by a double layer 

 of cells. This constitutes the cyst of the well-known Alcyonella- 

 larva, within which two polypides subsequently make their ap- 

 pearance by budding. In this budding both laminae of the cyst- 

 walls participate. The outer lamina serves for the formation of 

 the outer epithelium of the tentacles and the inner epithelium 

 of the alimentary canal ; while the central nervous system, which 

 in the larva is very large, is also most probably derived from it. 

 The inner lamina, on the other hand, forms all the muscles of the 

 body, as well as the genitalia and the inner epithelium of the body- 

 cavity. 



Nitsche had arrived at nearly the same conclusion regarding 

 the part taken by the two germinal laminse in the formation of the 

 tissues of the polypide in the m.arine polyzoon Flustra membra' 

 nacea f ; and he further % confirms Metschnikoff''s remarkable 

 observation regarding the reception of the eggs of Alcyonella 

 into a brood-capsule formed as a bud from the walls of the body- 

 cavity. He sees in this last observation a solution of the question 

 regarding the escape of the larvae from the body-cavity of the 

 parent, though no orifice which could serve as exit had been 

 hitherto detected. He has convinced himself that the brood-sac 

 of Alcyonella, which, quite like the polypide-buds, arises near the 

 invagination-orifice of the parent zooecium, finally opens at its 

 anterior end, where it is connected with the endocyst in the same 

 way as the tentacular sheath of a young polypide. The larva 

 * Bull, tie I'Acad. de St. Petersbourg, xv. 1871, p. 507. 

 t Zeitschr. f. wiss. Zool. Bd. sxi. p. 457. % Ibid. Bd. xxii. 



