The Origin of the Lung of Ampullaria. 99 



Those that were kept immersed drowned before the end of the second day, 

 but those that were kept in damp air or were permitted to climb out of the 

 water, remained in good health. The adults survive an immersion of a 

 month or more without injury, but they leave the water occasionally when 

 permitted to do so. 



The embryos that have been studied were washed out of the broken 

 shells by a stream of \vater, and they were fixed in picro-acetic acid or in 

 formalin and preserved in 70 per cent alcohol. After the shells were re- 

 moved by dilute nitric acid (8 to 15 drops in 100 ^c.cm. of alcohol) they 

 were cut into sections in three planes, transverse, horizontal, and sagittal, 

 a transverse section being one that is perpendicular to the long or principal 

 axis of the extended embryo ; horizontal, one that is parallel to the principal 

 axis and at right angles to the median plane of morphological symmetry; 

 sagittal, parallel to the principal axis and to the median plane of symmetry. 



Haidenhain's hematoxylin was used for staining on the slide, with eosin 

 as counterstain. Excellent results were obtained by shortening time. For 

 instance, the alum bath was used for one or two hours, and the hematoxylin 

 for only half an hour. Embryos that were to be mounted whole were stained 

 in acid borax carmine and cleared with clove oil. 



THE GENERAL ANATOMY OF THE EMBRYO. 



In order to make the account of the development of the organs of respi- 

 ration intelligible, it should be preceded by an outline sketch of the general 

 anatomy of the embryos. 



The anatomy of the embryo, at the time when the first traces of the 

 organs of respiration are found, will be understood from plate I, figure 7, 

 and plate 3, figure n. Plate I, figure 7, is a surface view of a young em- 

 bryo, and plate 3, figure n, a reconstruction of a series of oblique sagittal 

 sections of a slightly older one. Both are of the same length, 1.2 mm. 



The anterior region consists of a great head-vesicle, hv, and the foot, 

 /. In the ventral anterior region of the vesicle is a lipped opening, the 

 mouth, m. Above this, and encircling the vesicle, is a ciliated ridge, the 

 velum, v. The foot, f, is longer than broad, and its lower surface is flattened. 

 On its posterior border are a few large, clear cells, apparently calcareous, 

 three of which are shown in plate I, figure 7. In young embryos they are 

 large and numerous, but they disappear as growth progresses, and none 

 are to be found at the stage shown in plate 3, fig. n. On the dorsal surface 

 of the posterior region of the foot the ectoderm becomes thickened and 

 secretes a calcareous operculum, o. The posterior region of the body, 

 with the mantle, mantle-chamber, and shell, is shown in front view in plate 

 i, figure 7, and in section in plate 3, figure n. It resembles a bowl in shape, 

 with the inner concave surface facing to the right. The convex left-hand 

 surface is composed of the thickened ectoderm that secretes the shell, sh. 



