STAGES OF DEVELOPMENT 147 



the neck region the extraordinary size of the jugular vein is correlated with large di- 

 mensions of the head in comparison with the body. In the region of the abdomen the 

 dorsal aorta gives off two allantoic arteries. Of these the left artery is well developed 

 and the right is degenerating. A remarkable feature of the blood system at this stage is 

 that the primitive arrangement of a double aortic root is maintained. There is no trace 

 of the reduction in size, which takes place subsequently, of the left side of the root 

 (Plate V, fig. i). The duct of Botallus (db), i.e. the connective between the pulmonary 

 arch and the aortic roots, is also present on both sides. 



(iii) The excretory organ. The posterior segments of the opisthonephros have enlarged 

 and grown in a headward direction dorsal to the main mass of the excretory organ. The 

 character of the organ is therefore undergoing a change towards the purely metanephric 

 type found in the hatched bird. The metanephric duct — the ureter — is already dis- 

 tinguishable (Plate V, fig. I x), but the division of the organ at the level of the allantoic 

 artery (ala) does not mean a separation between the mesonephric and metanephric 

 portions. The two parts are in organic continuity dorsally and the division probably 

 foreshadows the groove that marks off" the fully developed metanephros into two lobes. 

 In this respect the kidney of adult penguins difters from that of the majority of birds in 

 which there are three lobes (Watson, 1883, p. 217). 



Up to this point the developmental stages have been covered by the bulk of the avail- 

 able material— the sixty-four Gentoo penguin embryos. For stages beyond it, right up 

 to the time of hatching, the additional fourteen Ring penguin embryos were at hand. 

 Fortunately the youngest of the latter coincided in development almost exactly with the 

 oldest of the Gentoo embryos. A comparison between the two of them after dissection 

 revealed no difl'erences in their anatomy. The extent of the permanent air sacs, for 

 example, in the smallest of the Ring penguin embryos, shown in Fig. i c, would serve 

 equally well to illustrate their condition in the Gentoo embryo (Plate IV, fig. i). 



8. The anatomy of the oldest of the Ring embryos, fledged in the coat of silky down 

 feathers with which it hatches, is represented on Plate VI, figs, i and 2. The first of the 

 figures shows a superficial and the second a deep dissection. The embryo is large, about 



150 mm. in length. 



The most noticeable difference between this stage and the preceding one, apart from 

 the fact that it is nearly double the size and has a complete covering of feathers, is the 

 inclusion of the yolk sac within the body-wall of the embryo. On the hind-limb the 

 claws are moreover uncovered, and the neonychia (ne) are wearing away their pomts. 

 Preparation for using the claws to destroy the embryonic membranes is therefore nearly 

 complete, and the appearance of the limb is in this respect in remarkable contrast with 

 the protected limb of the previous stage (Fig. 2). The horny bill with its sharp-pointed 

 egg tooth (et) has also undergone a great change in readiness for the act of hatching. 1 he 

 horn is deposited beneath the surface layer of the epidermis, the so-called epitrichium, 

 from a specialized zone of cells. It is exposed after the epitrichium is shed. The details 

 of the process and the histological changes leading up to it have been described from a 



