14 ALICE JOHNSON. 



skeleton of the limb than in the girdle, and more rapidly in the 

 axial than in the superficial regions of both skeletons. Its main 

 features are almost the same as those described by Strasser^ in 

 the developing cartilage of the newt. On the sixth day, or 

 thereabouts, the cells begin to be compressed in the direction of 

 the long axis of the cartilages. This happens especially in the 

 tibia and fibula. Dark irregularly-shaped masses — the " pre- 

 chondral elements" ofStrasser — appear among the cells. They 

 are apparently derived from the metamorphosed cells, for one 

 occasionally meets with forms that appear intermediate, in 

 which the protoplasm has become opaque and stains deeply, 

 while the nucleus is still visible. I take the prochondral ele- 

 ments to be cells which have retrograded still further and lost 

 their nuclei. 



Rather later, on the sixth or seventh day, the prochondral 

 elements have almost disappeared from the central part of the 

 cartilage. Their place is taken by a homogeneous, slightly- 

 staining matrix, by means of which the cells gradually become 

 widely separated from one another. Still later the cells take 

 on the crescent shape of adult cartilage cells. 



Morphology. — Since chicks of the same day vary so much 

 in their degree of development I have taken the length of the 

 hind limb as the standard of their age. The following table 

 shows roughly to what number of days of incubations these 

 lengths correspond : 



3ngtli of hind limb. 



Number of days of incubation, 



0-06 in.— 0-1 in. 



5—6 



0-12 in.— 0-2 in. 



6-7 



0-2 in.— 0-25 in. 



7-8 



0-25 in.— 0-3 in. 



8—9 



0-5 in.— 0-8 in. 



9-10 



1'5 in. — 3* in. 



14—20 



The Pelvic Girdle on its first appearance (length of hind limb 

 0'06 in. — see fig, 1) is seen in transverse sections to form one 

 mass with the skeleton of the limb. It consists of two slight 



• H. Strasser, " Zur Entwicklung der Extremitatenknoclien bei Salamandern 

 und Tritonen," ' Morph. Jahrbuch,' Band v, 1879. 



