ACALEPH^. 



109 



other, and frequently the apical tails of one are infixed in the cen- 

 tral interspace of the bodies of the preceding bundles ; and a chain or 

 string of bundles of spermatozoa are thus formed, which are easily- 

 detected by a moderate microscopic power. 



In Cyanaeas of an inch and a half in length, the males may be dis- 

 tinguished by the sperm sac in the plicated testis ; and the females by 

 the germinal vesicle and spot in the corresponding ovarium. But 

 the band-like genital organ in both is small, and the folds are indi- 

 cated by slight risings and depressions. 



The ovarium, like the testis, consists of a band with many folds 

 attached to the septum dividing the generative from the digestive 

 cavity. Its concave border is beset with similar tentacles ; but the 

 thin epithelium, on the under surface of the ovarium, is here and 

 there slightly ciliated, which has not been observed in the testis. 

 The tissue of the ovarium is looser, and it has more the aspect of a 

 cavity, than the testis. The minutest germs of the ova are nearest 

 that surface of the ovarium which is attached to the membranous 

 septum ; the most mature ova are on the opposite or free surface, 

 from which they project, covered only by a very thin membrane, and 

 giving it a coarse granular character. 



The ova at first consist of a germinal vescicle with its spot or 

 nucleus. They increase in size by the addition of a violet-coloured 

 yolk. In this state they are transferred from the ovarium to the 

 marsupial vesicles on the under surface of the arms ; but how they 

 get there is not known : they are doubtless impregnated " in transitu." 

 In the ova of the marsupial sacs, Siebold could no longer discern 

 the germinal vesicle, and he conceived, in conformity with the pre- 

 valent notion, that that important body had been destroyed as the 

 first eflFect of fecundation. No doubt its primitive character had been 

 altered and obscured by the cell-building processes which had ra- 

 diated from its nucleus, like those observed by Dr. M. Barry in the 

 ovum of the rabbit. 



The marsupial ova next assume an increase of size, and the yolk 

 begins to divide, by a spontaneous fission, which Dr. Siebold* describes 

 as commencing by a lateral indentation, as mjig. 56., which proceeds 

 36 57 38 59 60 61^:=^ G2 



across until the bipartition is complete, as in fig. 57. At all events, 



the vitelline mass is divided into two parts. Subsequent subdivisions, 



analogous to those of the ova of the Strongylus (p. 77.), are de- 



* Beitrage zur Naturgesch. der Wirbellosen Thiere, 4to. 1839. 



