20 



froiTi tlie scapula shows the IhicUeniiiii of tlie cells against the hone 

 fragment present in the following (not drawn) section. 



Embryo lacerta ag. G. (N. T. about 29) is distinguished from the 

 former one by a piece of bone which has grown larger in a ventio- 

 medial direction in the blastematic process of the clavicula, whi(di 

 has gi'own out in the same direction. The blasteuuitic cUuicular 

 process of the scapnlo-coracoid still reaches nnicli further veutrally 

 than the bone fragment. 



Embryo Lacerta ag. H. (N. T. about 80). Both the form and the 

 histological dilferentiation of the formation of the parts of the skelet- 

 on have undergone marked changes. The calcified cartilaginous 

 diaphysis is surrounded by a covering of perichondral bone. The 

 articulatio humeri is indicated by a layer of thick mesenchyme 

 which lies between the cartilaginous proximal humerus extremity 

 and the shoulder-girdle. Caitilage is found in the sca|)ulo-coracoid 

 in accordance with the position of the scapula and of the later 

 coracoideum s. str., i. e. in the dorsal and \enlro-caudal parts. The 

 venlro-cianial half consists of praechondrium and blastema, # except 

 that where, in the adult lizard, the epicoracoid, pj-ocoracoid and lig. 

 scapiilo procoracoideuni are found, we find in this end)ryo prae- 

 chondrium, and that at the place of the future fenestra [irincipalis and 

 of the membrana scapulo-procoracoidea, only thickened mesenchyme 

 blastema is found. Eoi' the rest (he scapulo has grown out in a 

 dorsal direction as well as the coracoid has done in widei- sense in 

 a ventral direction. The clavicular-process has grown longei' veutro- 

 medially, especially the bony nucleus lying in it. Moreover it is also 

 striking that caudal to the bony clavicle a compact blastema-mass 

 has developed. The significance of tliis will become |)lain laler. 



Embryo Lacerta ag. J. (N.T. about 31). The organs, the deve- 

 lopment -of which ai-e examined here, show no striking diti'erences 

 from embryo H. More cartilage is present in the scapnlo-coracoid 

 than before. The outline of the praechondrium against the blastema 

 is more easily definable. The blastema, of which the bony clavicle 

 occupies the cranial border, has increased in size but is still perfectly 

 homogeneous. It is distinctly loose of the coiacoid lying underneath 

 it (properly dorsal to it). The schemata of fig. H may now be 

 compared. 



Embryo Lacerta ag. L. (N.T. about 31). In this specimen many 

 of the parts still praechondral in the former embryo have become 

 cartilaginuos. The blastema, of which the bony clavicle occupies 

 the cranial border, has increased in .size, but is still homogeneous. 

 The bony clavicle is now neaily as long as the clavicular blastema. 



