(145) S. L. Schenk. "Beiträge z. Lehre v. d. Organanlage im motorischen Keimblatt." _Sitz. Wien. Akad._, Vol. LVII. 1860. (146) S. L. Schenk. "Beiträge z. Lehre v. Amnion." _Archiv f. mikr. Anat._, Vol. VII. 1871. (147) S. L. Schenk. _Lehrbuch d. vergleich. Embryol. d. Wirbelthiere._ Wien, 1874. (148) S. Stricker. "Mittheil. üb. d. selbstständigen Bewegungen embryonaler Zellen." _Sitz. Wien. Akad._, Vol. XLIX. 1864. (149) S. Stricker. "Beiträge zur Kenntniss des Hühnereies." _Wiener Sitzungsber._, Vol. LIV. 1866. (150) H. Virchow. _Ueber d. Epithel d. Dottersackes im Hühnerei._ Inaug. Diss. Berlin, 1875. (151) W. Waldeyer. "Ueber die Keimblätter und den Primitivstreifen bei der Entwicklung des Hühnerembryo." _Zeitschrift für rationelle Medicin._ 1869. (152) C. F. Wolff. _Theoria generationis._ Halæ, 1759. (153) C. F. Wolff. _Ueb. d. Bildung d. Darmcanals im bebrüteten Hünchen._ Halle, 1812. CHAPTER IX. REPTILIA. The formation of the germinal layers in the Reptilia is very imperfectly known. The Lizard has been studied in this respect more completely than other types, and there are a few scattered observations on Turtles and Snakes. The ovum has in all Reptilia a very similar structure to that in Birds. Impregnation is effected in the upper part of the oviduct, and the early stages of development invariably take place in the oviduct. A few forms are viviparous, viz. some of the blindworms amongst Lizards (Anguis, Seps), and some of the Viperidæ and Hydrophidæ amongst the Serpents. In the majority of cases, however, the eggs are laid in moist earth, sand, &c. Around the true ovum an egg-shell (of the same general nature as that in birds, though usually soft), and a variable quantity of albumen, are deposited in the oviduct. The extent to which development has proceeded in the oviparous forms before the eggs are laid varies greatly in different species. The general features of the development (for a knowledge of which we are mainly indebted to Rathke's beautiful memoirs), the structure of the amnion and allantois, &c. are very much the same as in Birds. The Lizards will be taken as type of the class, and a few noteworthy points in the development of other groups will be dealt with at the close of the Chapter. The following description, taken in the main from my own observations, applies to Lacerta muralis. The segmentation is meroblastic, and similar to that in Birds. At its close the resulting blastoderm becomes divided into two layers, a superficial epiblast formed of a single row of cells, and a layer below this several rows deep. Below this layer fresh segments continue for some time to be added to the blastoderm from the subjacent yolk. [FIG. 126. SECTIONS THROUGH AN EMBRYO OF LACERTA MURALIS REPRESENTED IN FIG. 129.