SUMMARY i6i 



papillae which make a transitory appearance on the dorsal side of the lung at stage 34 

 (Fig. I e, do). They are undoubtedly incipient air sacs which do not proceed with tlicir 

 development ; and in considering the possible reasons for their disappearance the whole 

 question of the function of air sacs in relation to an aquatic life arises. The ability to 

 give additional buoyancy to their possessor would be of great value to a surface-swim- 

 ming animal such as an aquatic reptile. It would ensure rapid return to the respiratory 

 medium after fishing in moderate depths and, in addition, supply a necessary air reservoir 

 to enable the animal to stay beneath the surface for a considerable time. As soon as the 

 muscular effort of the pursuit of prey under water relaxed, the main lung — with the 

 considerable assistance of the air sacs — would operate in bringing the predator to the 

 surface. In doing this, however, any air sacs on the dorsal side of the lung would be 

 hemmed in between the distended lung and the unyielding dorsal wall of the coelomic 

 cavity, and in time — their usefulness being unexploited — they would be eliminated. 

 This, perhaps, is the meaning of the appearance of dorsal papillae on the lung of a 

 developing penguin for a brief time, and if so, they may be accepted as evidence that the 

 bird developing them is in fact a primitive form. 



SUMMARY 



The development of Pygoscelis has been described from a very complete series of 

 embryos of two species, Pygoscelis papiia and Pygoscelis antarctico. The material con- 

 sisted of a collection of 78 embryos with the following characteristics : 



Number of 

 Species Characteristics specimens 



Pygoscelis papua Open medullary folds 2 



Incomplete amnion 19 



Limb rudiments just apparent 8 



Head flexure maximal 10 



Beak rudimentary iS 



Feather papillae restricted in distribution 6 



Feather rudiments uniformly distributed I 



Pygoscelis antarctica Feather rudiments uniformly distributed i 



Fully fledged in pre-natal down 13 



The developmental stages have been correlated with the standard stages of the fowl 

 as fixed by Keibel and Abraham's Normentafel. 



The general anatomy of each of the stages has been examined and the more out- 

 standing features reported. Particular attention has been paid to the following topics: 

 the development of the lungs and air sacs ; the brain ; the feathers ; the cartilaginous 

 skeleton; the heart. Other special features to which shorter reference has been made 

 include: (i) eye structure with reference to the pecten, (ii) the skin and claws, (iii) the 

 digestive organs, (iv) the organs of excretion. 



The close similarity between the course of development of the viscera in the penguin 

 as compared with the fowl has been demonstrated. Where differences exist they are 

 differences of detail only (e.g. the double aortic root is persistent for a longer time; the 



