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 development, 

 with the exception of the very first stages, takes place 

 externally. 



In many of the viviparous forms (Mustelus, Galeus, Car- 

 charias, 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 spinax. 



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

 danus, Acanthias, Scymnus, Galeus, Squalus, Mustelus, Carcharias, Sphyrna, 

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

 tiurus, Cestracion, Raja 1 . 



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 2 . The greater part of it is formed 

 of peculiar oval spherules of food-yolk, held together by a 

 protoplasmic network. The protoplasm is especially concen- 

 trated in a small lens-shaped area, known as the germinal disc, 

 which is not separated by a sharp line from the remainder of 



1 For further details, vide Miiller (No. 48). - Vide Vol. n., p. 62. 



