156 DISCOVERY REPORTS 



The pectoral arch has added to it a sternum and the beginnings of sternal ribs. The 

 pelvic girdle is built up by the union of three separate cartilages, the rudiments of the 

 ilium, ischium, and pubis respectively. Owing to the position of the limb this point is 

 not clear in Fig. 7 c, and the girdle has therefore been drawn separately in Fig. 7 d. In 

 penguins, as in other birds, the pubis takes part in the formation of the acetabulum 

 (Mannich, 1903, p. 29), and it is clear that it rotates in development into a position 

 parallel with the ischium, in the manner discussed by Goodrich (1930, pp. 206-12). 



Between stages 31 and 34 (Fig. 7 e) no great changes occur in the cartilages other than 

 growth. A few additions are made to their number. In the head, for example, the supra- 

 occipitals make their appearance just dorsal to the otocyst, and the articular separates 

 from Meckel's cartilage. Two hyoid cartilages are developed under the lower jaw close 

 beneath the articular. They are just visible in stage 31, but they keep pace with the 

 development of the tongue and are not prominent while the beak is small. The pre- 

 maxilla becomes shifted forwards during the active growth of the beak so as to be 

 separated from the mesethmoid. It takes the external nares with it, and in the space 

 developing between it and the mesethmoid a number of frail ethmoid cartilages are 

 laid down. 



Most of the cartilages of the appendicular skeleton are complete by stage 34. In the 

 flipper the first digit is represented, and in the hind-foot a small two-jointed hallux 

 grows on the inner side. The metatarsals of digits 2, 3, and 4 are quite separate and, 

 owing to the position of the limb, the normal tendency of the fibula to cross over from 

 the outer to the inner side of the leg is accentuated. 



Leaving the Victoria blue material one other preparation is figured with a view to 

 showing the cartilages, especially of the limbs. This specimen (Plate V, fig. 2) was the 

 last of the Gentoo embryos. The cartilages were exposed by peeling off the skin and 

 muscles, a task rendered easier because the tissues were rather brittle after fixation in 

 corrosive acetic. Two striking features of the cartilages at this stage are (i) the great 

 breadth of the scapula, and (ii) the firmness and extent of the completed pelvic girdle. 



The broad scapula is especially characteristic of the genus Pygoscelis (Watson, 1883, 

 p . 25) but, in comparison with normal flying birds, all penguins have an exceptionally wide 

 shoulder blade. This of course is a well-known feature of the skeleton of many other 

 aquatic vertebrates and may almost be regarded as an indication of the use of the fore- 

 limbs as organs of propulsion through water. The powerfully developed pelvic girdle of 

 these birds is no doubt correlated with their upright gait on land. It is common know- 

 ledge that they prefer this slow method of progression to lying prone and moving fast 

 with the help of the wings as well as of the legs. Other points of interest about the 

 skeleton include : (i) the large size of the patella, (ii) the early development of the uncinate 

 processes of the ribs, (iii) the presence of two extra sesamoids, just indicated in the 

 swollen brachial tendon, (iv) the commencement of the replacement of the cartilages 

 by bone (indicated in the figure by the little granular areas in the centre of the long 

 cartilages), and (v) the advanced state of the development of the clavicle. This latter of 

 course is not preformed in cartilage at all: it is not a replacement bone, and it is re- 



