[11] Note to Teachers.—A general dissection of a frog should be made and exhibited to the class. Detailed instructions for such a dissection will be found in Marshall’s The Frog (Smith, Elder & Co.) or in Huxley and Martin’s Elementary Practical Biology (Macmillan). The frog’s heart continues to beat for some time after the death of the animal, and may be shown as an illustration of the next section of this chapter. Teachers who are unskilled in dissection may obtain prepared dissections, mounted skeletons, etc., from Newmann & Co., 84 Newman Street, London, W. [12] More detailed instructions will be found in Foster and Shore’s Physiology for Beginners (Macmillan). [13] The Descent of Man. Cheap edition, 1s. (Murray). [14] Gilbert White, The Natural History of Selborne. [15] See Lloyd Morgan’s Habit and Instinct (Arnold), Thompson-Seton’s Wild Animals I have known (Nutt), and Long’s School of the Woods (Ginn). [16] Darwin, The Origin of Species (Murray). Cheap edition, 1s. [17] The amnion originates, early in the second day, as a double fold of the yolk-surface in front of the embryo. Similar folds arise round the sides and tail, forming a low wall (Fig. 187, A); the folds gradually grow over the embryo (Fig. 187, B) until, during the fourth day, they meet (Fig. 187, C) and enclose it in a protective transparent bag, containing a watery fluid. Fig. 187.—Diagrams to illustrate the method of development of the amnion. The embryo and the rest of the yolk are supposed to be seen in median longitudinal section; the head end is to the right. For convenience, the yolk-membrane (yk.m.) is represented at some distance from its contents. am, folds of amnion; am.c., amniotic cavity; em, embryo; yk, yolk; yk.st., yolk-stalk. (After Foster and Balfour.) [18] I desire to acknowledge my indebtedness, in drawing up this section, to Prof. Lloyd Morgan’s Habit and Instinct (Arnold). [19] Lloyd Morgan’s Habit and Instinct (Edward Arnold). [20] Lloyd Morgan’s Habit and Instinct (Edward Arnold). [21] Our Bird Friends (Cassell). [22] The Natural History of Selborne. [23] The Natural History of Selborne. [24] A delightful account is given in the story of “Silverspot” (Thompson-Seton’s Wild Animals I Have Known). [25] Latin, passer, a sparrow. [26] The Cambridge Natural History, Vol. VIII. (Macmillan.) [27] Marshall’s The Frog (Smith, Elder & Co.). [28] Greek: orthos, straight; pteron, a wing. [29] See Footnote, p. 372. [30] Greek: koleos, a sheath; pteron, a wing. [31] Living eggs, larvae, and pupae of Lepidoptera may be obtained from Mr. H. W. Head, Burniston, near Scarborough, and other dealers. [32] Greek: lepis, a scale; pteron, a wing. [33] Injurious and Useful Insects (Bell).