OVUM. 



of sexual union, and they are thence taken up 

 singly by the open upper extremities of the two 

 oviducts ; through the whole of which canals 

 they descend, and in their passage receive an 

 additional covering of a peculiar gelatinous 

 or albuminous substance, which adheres 

 closely to the surface of the yolk membrane, 

 and is firm and of comparatively little bulk 

 while the ova are still within the oviduct, but 

 which after exclusion rapidly. swells by the im- 

 bibition of a large quantity of water, so as to 

 become several times its original thickness, 

 and to assume a soft gelatinous consistence. 



The ova which have passed through the 

 oviducts remain for a time accumulated in large 

 numbers in a dilated part of the canals near 

 their lower end, until the whole or greater 

 part of those which are ready to descend 

 from the ovary have passed down ; and then, 

 while the male still continues united with the 

 female, the ova are rapidly excluded, and the 

 male sheds the spermatic fluid in abundance, 

 partly on the ova as they pass into the water, 

 and partly after separating from the female, 

 upon the spawn as it floats in the water. 

 The importance of the imbibition of water by 

 the gelatinous covering immediately on the 

 exclusion of the ova and just at the time when 

 the spermatic fluid has been placed upon 

 them, in securing the access of the sperma- 

 tozoa to the surface of the vitelline membrane 

 through the stiff jelly, and in thus promoting 

 fecundation, will be afterwards more particu- 

 larly adverted to. 



In the tailed Amphibia, such as the differ- 

 ent kinds of Newt (Salamandra, Triton, and 

 Lissotriton) there is not the same union of 

 the male and female as in the tailless or Anu- 

 rous Batrachia ; and impregnation takes place 

 by the entrance of the spermatic fluid, shed in 

 the water by the male while placed near the fe- 

 male, into the oviducts of the latter. In these 

 animals the external covering consists of an 

 elliptical membranous capsule filled with a 

 clear fluid and containing the coloured sphe- 

 rical yolk ; but there is also externally a 

 small quantity of gelatinous substance which 

 in some of them serves to attach the ova 

 to leaves of plants orjrther objects. In the 

 common larger and smaller newts the ova 

 are in smaller number than in the frog or 

 toad, and are excluded one by one by the 

 female, which deposits them singly in a 

 folded leaf or other place of security.* 



The yolk of the ripe ovarian ovum in Am- 

 phibia consists of a thick opaque mass of 

 vitelline substance, within which and towards 

 one side the large germinal vesicle is placed. 

 The vitelline substance is usually of a darker 

 colour on the exterior and lighter in the cen- 

 tre. In the common toad the superficial part 

 is almost black ; in the common frog, Rana 

 tcmporaria, it is of a very dark brown ; and it is 

 in different other species of various hues, as 



* I have frequently observed this process, which 

 has been beautifully described and figured by 

 Mauro Rusconi in his work, " Amours des Sala- 

 mandres Aquatiques, Milan, 1821." 



stated in a former part of this article. The 

 dark superficial part does not in general cover 

 the whole surface of the yolk, but is deficient 

 on one side ; and its extent as compared with 

 the inner paler part, which appears where the 

 dark part terminates, varies in different spe- 

 cies and is greater in proportion to the de- 

 gree of advancement of the ova. In some 

 species, as in the Alytes obstetricans, of which 

 C. Vogt has given an excellent description-}-, 

 it does not, when the ovum is mature, occupy 

 more than a half of the surface ; but in the 

 common frog and toad it covers so much of 

 the surface of the yolk when it is about to 

 leave the ovary, that the gray internal part 

 is only seen as a defined round spot on the 

 opposite side. In the undeveloped ovarian 

 ova, however, the dark part is much more 

 limited in its extent, thus allowing a greater 

 part of the lighter-coloured internal part to 

 be seen. 



Fig. 64 *. 



Ovum of the Frog. 



a. (From Newport.') An ovum of the frog half an 

 hour after impregnation, covered with its gelatinous 

 mass. The dark part of the egg or yolk is seen to 

 be surrounded by a vitelline membrane. Sperma- 

 tozoa were seen everywhere in the gelatinous en- 

 velope, but are not represented in the figure. 



b. Vertical section of the yolk or ovarian ovum 

 of the frog which has been hardened in alcohol, 

 showing the germinal vesicle within and the canal 

 of the yolk which leads down to it from the upper 

 or germinal pole. The external line indicates the 

 vitelline membrane. 



c. Diagrammatic representation of the same sec- 

 tion, showing, according to the views of Ransom, 

 the relation of the canal or depression of the yolk 

 to the germinal vesicle. The micropyle, if it 

 exists in these ova, may be situated in the vitelline 

 membrane immediately above this canal. This 

 figure also shows the relations of the dark and light 

 coloured parts of the yolk substance. 



f Entwickelungsg. der Geburtshoelferkrote, Solo- 

 thurn, 1842.. 



