CHAPTER III. 

 ELASMOBRANCHII. 



The impregnation of the ovum is effected in the oviduct. In most 

 forms the whole of the subsequent development, till the time when 

 the embryo is capable of leading a free existence, takes place in the 

 uterus; but in other cases the egg becomes enveloped, during its 

 passage down the oviduct, first in a layer of fluid albumen, and finally 

 in a dense horny layer, which usually takes the form of a quadrilateral 

 capsule with characters varying according to the species. After the 

 formation of this capsule the egg is laid, and the whole of the develop- 

 ment, with the exception of the very first stages, takes place exter- 

 nally. 



In many of the viviparous forms (Mustelus, Galeus, Carcharias, 

 Sphyrna) the egg is enclosed, during the early stages of development 

 at any rate, in a very delicate shell homologous with that of the 

 oviparous forms ; there is usually also a scanty albuminous layer. 

 Both of these are stated by Gerbe (No. 42) to be absent in Squalus 

 spin ax. 



The following are examples of viviparous genera: Hexanchus, Noti- 

 danvis, Acanthias, Scymnus, Galeus, Sqitalus, Mustelus, Carcharias, Sphyrna, 

 Squatina, Torpedo; and the following of oviparous genera: Scyllium, Pris- 

 tiurus, Cestracion, Raja'. 



The ovum at the time of impregnation has the form of a large 

 spherical mass, similar to the yolk of a bird's egg, but without a 

 vitelline membrane ^ The greater part of it is formed of peculiar 

 oval spherules of food-yolk, held together by a protoplasmic network. 

 The protoplasm is especially concentrated in a small lens-shaped area, 

 known as the germinal disc, which is not separated by a sharp line 

 from the remainder of the ovum. Yolk spherules are present in this 

 disc as elsewhere, but are much smaller and of a different character. 

 The segmentation has the normal meroblastic character (fig. 15) and 

 is confined to the germinal disc. Before it commences the germinal 



1 For further details, vide Miiller (No. 48). ^ Vide Vol. i., p. 50. 



B. E. II. 3 



