594 ANATOMY OF VERTEBRATES. 



this period the ova in Osseous Fishes escape into the cavity of 

 the ovarium, and the ectosac then receives its villi, or appendages 

 for adhesion, in the Fishes possessing them. The ovisac remains 

 behind, and coalesces with the stroma of the ovigerous layer, to 

 form, according to Barry, a ' vesicle analogous to the Graaffian 

 vesicle of Mammals;' 1 but the evacuated ovisacs collapse and 

 speedily disappear, after the discharge of the ova, in the shrunken 

 ovarium of Dermopteri and in the collapsed ovarian bag of Osseous 

 Fishes : they are longer recognisable in the more compact and 

 solid ovaries of the Plagiostomes. 



The earlier stages of the developement of the ovum within the 

 ovisac are illustrated in figs. 1 a-d (pp. 1 and 2), from Ransom's 

 observations on the Stickleback ( Gasterosteus aculeatus). In 1 A 

 the ovisac, c, has a diameter of -^fa inch: the germinal vesicle, d, 

 appears as a gelatinous spheroid, with few 'macula?' and a scarcely 

 definite wall : a slightly turbid fluid, a, fills the space between 

 the vesicle and the ovisac, from the inner surface of which a few 

 delicate epithelial cells, b, project. In b, with a diameter of 

 -y^-q inch, the macula? have increased in number, and the germinal 

 vesicle in size : fine yolk-granules have begun to aggregate near 

 the periphery, but there is no vitelline membrane : on the exterior 

 of the ovisac are the nuclei of flattened cells. In c, with a 

 diameter of j-^ inch, the macula? have become more numerous 

 and distinct : the yolk-granules more abundant and opake : the 

 yolk-mass is now more circumscribed, and a clear space intervenes 

 between it and the ovisac. In D, with a diameter of T ^ inch, 

 the germinal vesicle has more macula?, but has ceased to grow : 

 the yolk-granules are much increased in number, and a clear 

 peripheral space indicates the beginning of the formation of the 

 external membrane, ch. 



The ' roe,' or ova, of Osseous Fishes, being usually shed in or 

 on the sediment of shores, are subject to the action of the flow of 

 streams or break of waves upon their sandy or gravelly bed, and 

 these pellucid and seemingly delicate vesicular spherules are 

 accordingly provided with a very elastic and resisting outer coat, 

 constructed on a principle analogous to that by which a tooth 

 sustains and exerts its pressure. This coat is composed of a 

 close-set series of hollow columns, set perpendicularly to the sur- 

 face, fig. 416, h : the part of the outer surface turned into view, at 

 c, shows the pores, or ' lumina,' of the tubules. The yolk, a, with 

 its granules, granular and nucleated corpuscles, and oil-globules, 

 now shows its delicate structureless membrane, which is partly 



1 cix 1st Series, p. 814. 



