Lilian V. Sampson 475 



2. Embryonic Stages. 



Among the embryonic stages, I have material fcr a more complete 

 history, especially in the later development, than the egg-stages have 

 shown. For convenience in naming the stages, I have nnmbered them 

 in order, although, owing to some breaks in the series, consecutive num- 

 bers do not represent equal advances in development. Since the ages of 

 the preserved eggs are not Ivnown, I have compared the eggs with figures 

 of embryos that were watched during development, and whose age was 

 noted. The comparison shows that stages VI, XIII, XIV, and XV, 

 correspond nearly with the embryos under observation which were labelled 

 " 7-8 days, about 13 days, just hatched, one hour old." As I have indi- 

 cated, the former accounts of Hylodes have shown that the tree-frog 

 at no time exists as a free-swimming larva, but that it develops enclosed 

 within a membrane, which is not ruptured until the frog has reached a 

 stage essentially like the adult, though sexually undifferentiated. When 

 the gelatinous secretion about the preserved eggs has been removed, the 

 vitelline membrane is found to be a tough transparent envelope through 

 which the embryo can readily be seen. The eggs are of great size owing 

 to a large amount of yolk, and are at first unpigmented. As the small, 

 colorless embryo develops, it encircles the yolk more and more, while, at 

 the same time, the yolk diminishes in quantity. 



In stage IV (Figs. 8, 9), the diameter of the egg preserved in alcohol 

 measures a little less than 3 mm., and the embryo is about one-half the 

 length of the circumference. Both pairs of legs are present as knobs 

 at the sides of the embryo; the tail also is scarcely more than a knob, 

 although slightly lengthened and turned to the right or to the left side. 

 In this stage the tail usually covers the anal opening, though the pos- 

 terior rim of the anus still shows on the side from which the tail is 

 turned away. The primary divisions of the brain are seen, the auditory 

 vesicles are very distinct, and the optic cups are discernible. The embryo 

 lies flat upon the egg so that its contour is scarcely raised above the 

 level of the yolk. In stage III, the tail and legs are shorter than in 

 stage IV, and on either side of the neck of the embryo are four deep 

 parallel grooves, the ectodermie depressions which lie opposite the vis- 

 ceral pouches. In stage II (Fig. 7), the anus is seen at the posterior 

 extremity of the embryo, and the tail is not formed, but the rudiments 

 of both pairs of legs appear at about this stage. The optic vesicles show 

 as diverticula from the brain. In stage I (Fig. 6), the blastopore is just 

 closing, the divisions of the brain are not seen, but the head end of the 

 embryo is broader than the trunk. 



