(161) H. Dutrochet. "Recherches s. l. enveloppes du foetus." _Mém. d. Soc. Méd. d'Emulation_, Paris, Vol. VIII. 1816. (162) W. K. Parker. "On the skull of the common Snake." _Phil. Trans._, Vol. 169, Part II. 1878. (163) H. Rathke. _Entwick. d. Natter._ Königsberg, 1839. _Chelonia._ (164) L. Agassiz. _Contributions to the Natural History of the United States_, Vol. II. 1857. _Embryology of the Turtle._ (165) W. K. Parker. "On the development of the skull and nerves in the green Turtle." _Proc. of the Roy. Soc._, Vol. XXVIII. 1879. _Vide_ also _Nature_, April 14, 1879, and _Challenger Reports_, Vol. I. 1880. (166) H. Rathke. _Ueb. d. Entwicklung d. Schildkröten._ Braunschweig, 1848. _Crocodilia._ (167) H. Rathke. _Ueber die Entwicklung d. Krokodile._ Braunschweig, 1866. CHAPTER X. MAMMALIA. The classical researches of Bischoff on the embryology of several mammalian types, as well as those of other observers, have made us acquainted with the general form of the embryos of the Placentalia, and have shewn that, except in the earliest stages of development, there is a close agreement between them. More recently Hensen, Schäfer, Kölliker, Van Beneden and Lieberkühn have shed a large amount of light on the obscurer points of the earliest developmental periods, especially in the rabbit. For the early stages the rabbit necessarily serves as type; but there are grounds for thinking that not inconsiderable variations are likely to be met with in other species, and it is not at present easy to assign to some of the developmental features their true value. We have no knowledge of the early development of the Ornithodelphia or Marsupialia. The ovum on leaving the ovary is received by the fimbriated extremity of the Fallopian tube, down which it slowly travels. It is still invested by the zona radiata, and in the rabbit an albuminous envelope is formed around it in its passage downwards. Impregnation takes place in the upper part of the Fallopian tube, and is shortly followed by the segmentation, which is remarkable amongst the Amniota for being complete. Although this process (the details of which have been made known by the brilliant researches of Ed. van Beneden) has already been shortly dealt with as it occurs in the rabbit (Vol. II. p. 98) it will be convenient to describe it again with somewhat greater detail. The ovum first divides into two nearly equal spheres, of which one is slightly larger and more transparent than the other. The larger sphere and its products will be spoken of as the epiblastic spheres, and the smaller one and its products as the hypoblastic spheres, in accordance with their different destinations.