486 



On the Development of Apteryx. [Mar. 22, 



regions. The resemblance at this stage to the shoulder- girdle of the 

 ostrich is very close, but the late occurrence of the distinction between 

 coracoid and procoracoid, and their formation by fenestration of a 

 continuous cartilage, are remarkable. In Stage K the procoracoid 

 has degenerated into a ligament which now forms the sole pre- axial 

 boundary of the coracoid fenestra. Later, the coracoid ossification 

 extends pre-axiad until the membrane of the fenestra is replaced by 

 bone, but even in the adult the position of the fenestra is marked by 

 the thin, often emarginate plate which forms the inner or pre-axial 

 portion of the coracoid. The small aperture situated mesio-ventrad 

 of the glenoid cavity, and sometimes described as the coracoid 

 fenestra, serves for the transmission of a nerve. 



In Stage E the manus contains three well-chondrified sub-equal 

 digits ; the carpals are not yet chondrified, and are only indicated by 

 a concentration of nuclei in the blastema. In Stage F the second 

 digit has increased out of all proportion to the first and third. In 

 Stage G the pollex has degenerated, and its position is indicated only 

 by a concentration of nuclei in the mesoblast; two well-marked 

 carpals have appeared, one of which — the radiale — lies pre-axiad and 

 slightly proximad of the other, which gives attachment to the second 

 and third metacarpals. In Stage H the radiale lies entirely proximad 

 of the second or distal carpal, which is closely applied to the proximal 

 ends of the two metacarpals. In the newly-hatched bird the second 

 and third metacarpals have ankylosed with one another and with the 

 distal carpal, the radiale remaining separate. 



In Stage Gr the pubis closely resembles that of a chick of the 

 6th day, the pubis being vertical and the ilium comparatively short. 

 In Stage E the ilium has lengthened greatly, and the pubis forms an 

 angle of 20° with the vertical. In Gr the adult form is assumed, and 

 in H ossification has begun. 



In Stage D the tarsus consists of three elements, tibiale, fibulare, 

 and a single cartilage representing the five distalia. Besides the 

 three functional digits of the adult, and the pollex, which at this 

 stage has its normal connexion with the tarsus, there is a distinct 

 vestige of the fifth metatarsal in the form of a rod-like cartilage, 

 0'4 mm. long. In Stage E the foot has elongated ; the pollex has 

 shifted distalwards, and is now attached to the pre-axial edge of the 

 second metatarsal at about the middle of its length. The fifth meta- 

 tarsal is still distinct, but has not increased in size ; the tibiale and 

 fibulare have united. 



In Stage E the foot has nearly attained its adult form. The united 

 tibiale and fibulare instead of being, as in the preceding stage, in 

 close contact with the combined distalia, are separated from them by 

 a narrow in-growth of connective tissue, the rudiment of the meso- 

 tarsal semilunar pad. The fifth metatarsal is still visible, but has 



