OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 243 



6. The evidence from embryology is in favour of the early divergence of the 

 Aptenodytidse from the main stem of the flying birds, and of the common ancestor of 

 both having been a flying bird, though of a clumsy type. 



7. We were unable to verify the facts recorded by other observers in the ossification 

 of the limbs, as we were handicapped by the decalcifying action of formalin even in 

 weak solutions, so that our observations in this direction could not be considered 

 altogether satisfactory. 



ILLUSTRATIONS. 

 Plate I. 



Fig. 1. Gentoo penguin, embryonic area, early stage. 



Fig. 2. Gentoo penguin, within first 24 hours, showing head fold, medullary folds, and primitive streak. 

 This stage resembles that of the chick at 22 hours. 



Fig. 3. Gentoo penguin, said to be 3 days. This stage of development resembles the chick at 24 hours. 



Fig. 4. Gentoo penguin, said to be at 5 days. The stage of development resembles the chick at 26 hours. 

 The photograph shows specially the cephalic flexure, the hinder ends of the medullary folds enclosing the 

 bulbous extremity of the notochord, and the remains of the primitive streak. 



Fig. 5. Gentoo penguin, said to be 6 days. Comparable to the chick at 40 hours. The photograph 

 shows the bulbous expanded posterior end of the notochord, and evidence of a segmentation of the neural 

 folds. 



Fig. 6. Gentoo penguin, about 7 days. 



Plate II. 



Fig. 7. Duck embryo of 5 days. Compare fig. 4 of penguin of same age. 



Fig. 8. Duck embryo of 6 days. Compare fig. 5 of penguin of same age. 



Fig. 9. Duck embryo of 7 days. Compare fig. 6 of penguin of same age. 



Fig. 10. Gentoo penguin embryo of 12 days. Both upper and lower limbs have appeared on the surface 

 of the trunk. 



Fig. 11. Gentoo penguin embryo of 13 days. 



Fig. 12. Duck embryo of 8 days. Compare character of limbs with those of penguin of 13 davs 

 (fig. 11). 



Figs. 13, 14, and 15. Successive stages of Gentoo penguin embryos at 15, 16 and 17 daysj to illustrate 

 stages in the development of the sickle-shaped fore limb of the adult. 



Fig. 16. Duck embryo of 12 days, in situ, to show how the embryonic attitude is maintained, and is not 

 dependent upon external pressure. 



Fig. 17. Gentoo penguin embryo of 22 days, to show advancing change in the position of the feet. 

 Compare figs. 15 and 23, which show other stages in the development of the peculiar flat base of the adult. 



Plate III. 

 Fig. 18. Duck embryo of 13 days. 



Figs. 19 and 20. Gentoo penguin embryo of 18 and duck embryo of 12 days, photographed to the same 

 scale, to show altering characters of the limbs, when the trunks are of the same size. 

 Fig. 21. Duck embryo of 14 days. 



Fig. 22. Duck embryo of 15 days. Compare with fig. 15 of gentoo penguin embryo of the same a«e 

 TRANS. ROY. SOC. EDIN, VOL. XLVII. PART II. (NO. 10). 36 ° 



