x, x¹, connectives between collateralis colli and carotid. y, artery to shoulder and skin. z, z¹, z², branches of common carotid. 1¹, 2¹, 3¹, branches of ischiadic artery. CHAPTER IX THE DEVELOPMENT OF THE ALLIGATOR (_A. mississippiensis_) INTRODUCTION With the exception of S. F. Clarke’s well-known paper, to which frequent reference will be made, practically no work has been done upon the development of the American alligator. This is probably due to the great difficulties experienced in obtaining the necessary embryological material. Clarke, some twenty years ago, made three trips to the swamps of Florida in quest of the desired material. The writer has also spent parts of three summers in the Southern swamps--once in the Everglades, once among the smaller swamps and lakes of central Florida, and once in the Okefinokee Swamp. For the first of these expeditions he is indebted to the Elizabeth Thompson Science Fund; but for the more successful trip, when most of the material for this work was collected, he is indebted to the Smithsonian Institution, from which a liberal grant of money to defray the expenses of the expedition was received. The writer also desires to express his appreciation of the numerous courtesies that he has received from Dr. Samuel F. Clarke, especially for the loan of several excellent series of sections, from which a number of the earlier stages were drawn. In preparing the material several kinds of fixation were employed, but the ordinary corrosive sublimate-acetic mixture gave about the most satisfactory results. Ten per cent. formalin, Parker’s mixture of formalin and alcohol, etc., were also used. In all cases the embryos were stained _in toto_ with borax carmine, and in most cases the sections were also stained on the slide with Lyon’s blue. This double stain gave excellent results. Transverse, sagittal, and horizontal series of sections were made, the youngest embryos being cut into sections five microns thick, the older stages ten microns or more in thickness. THE EGG FIGURES 1, 1_a_ (PLATE VI.) The egg (Fig. 1) is a perfect ellipse, the relative lengths of whose axes vary considerably in the eggs of different nests and slightly in the eggs of the same nest. Of more than four hundred eggs measured, the longest was 85 mm.; the shortest 65 mm. Of the same eggs, the greatest short diameter was 50 mm.; the least short diameter was 38 mm. The average long diameter of these four hundred eggs was 73.74 mm.; the average short diameter was 42.59 mm. The average variation in the long axis of the eggs of any one nest was 11.32 mm., more than twice the average variation in the short axis, which was 5.14 mm. No relation was noticed between the size and the number of eggs in any one nest. Ten eggs of average size weighed 812 grams--about 81 grams each.