(35) W. Salensky. "Ueber die Knospung d. Salpen." _Morphol. Jahrbuch_, Bd. III. 1877. (36) W. Salensky. "Ueber die Entwicklung d. Hoden u. über den Generationswechsel d. Salpen." _Zeit. f. wiss. Zool._, Bd. XXX. Suppl. 1878. (37) C. Semper. "Ueber die Entstehung d. geschichteten Cellulose-Epidermis d. Ascidien." _Arbeit. a. d. zool.-zoot. Instit. Würzburg_, Vol. II. 1875. (38) Fr. Todaro. _Sopra lo sviluppo e l'anatomia delle Salpe._ Roma, 1875. (39) Fr. Todaro. "Sui primi fenomeni dello sviluppo delle Salpe." _Reale Accademia dei Lincei_, Vol. IV. 1880. 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, 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 spinax. The following are examples of viviparous genera: Hexanchus, Notidanus, Acanthias, Scymnus, Galeus, Squalus, Mustelus, Carcharias, Sphyrna, Squatina, Torpedo; and the following of oviparous genera: Scyllium, Pristiurus, Cestracion, Raja[17]. [17] For further details, _vide_ Müller (No. 48).