302 LECTURE XII. 



prevent the contact of the milt. The ovarian, or abdominal aperture 

 of the oviduct is free and distinct from the ovary itself in the Pla- 

 giostomes, the Lepidosiren, the Sturgeons, and Polypterus. If a 

 little powdered charcoal be sprinkled on the ovarian orifices and 

 ligaments exposed by opening the abdomen in a fresh caught female 

 Dog-fish, the particles will be seen to move towards and enter 

 the common oviducal aperture, indicative of a ciliated epithelium 

 in the serous membrane, which may aid in the transport of the 

 ova to that aperture. 



There is reason to suppose that impregnation of the eggs of both 

 Sharks and Rays takes place in the ovarium or the contiguous part of 

 the oviduct ; for they become enveloped in the dense albuminous 

 secretion of the nidamental glands after having passed that part, which 

 covering would prevent the subsequent influence of the spermatozoa. 

 The form of the egg, when thus invested, is remarkable, and very dif- 

 ferent in diiferent genera of Plagiostomes. In the Skate the ovum is 

 an oblong, four-sided, flattened case, with the angles produced 

 forwards and backwards, like those of a butcher's tray (prep. 3235). 

 The embryo skate is packed in the cavity with its broad pectorals 

 bent upon its back, and its tail coiled round the body : the vitellicle 

 is appended by a short and contracted neck. In the spotted Dog- 

 fish {ScylUum, No 3249) the ova are also quadrilateral, but longer, 

 and the angles are extended into long filamentary tendrils, which 

 attach themselves to floating sea-weed, and thus keep the ovum near 

 the surface, where the influence of solar heat and light is greatest 

 (Jig. SI). The eggs of the Calovhynchus them- 

 selves resemble some broad-leafed fucus, and 

 thus, probably, deceive the fish that might other- 

 wise devour them : they are in the form of a long 

 depressed ellipse, with a broad plicated and 

 fringed margin (No. 3235, A and b). The large 

 Shark's ovum (prep. 3245), resembles that of the 

 ScylUum, with the addition of a series of trans- 

 verse parallel ridges crossing the anterior and 

 posterior surfaces. An elongated pyriform shell 

 of a plagiostomous ovum, transmitted to me from 

 Australia, as that of the Cestracion, is charac- 



Egg and Embryo ; Scylltuni. 



One fourth nat. size. tcriscd by a broad ridge or plate, which is wound 

 edge-wise around the ovum in five spiral circumvolutions. The sub- 

 stance of all these egg-coverings is a light, but firm, albuminous horn- 

 like tissue, of a more or less deep brown colour : the orifices {Jtg. 

 81. c) admit the sea-water to the pendant respiratory filaments of 

 the inclosed embryo (a). The yolk-bag is shown at b. 



The essential difference between the oviparous and ovo-viviparous 



