434 



Special Vertebrate Organogenesis 



it is adequate for describing development, 

 and it keeps one from using the term "organ- 

 izer" indiscriminately and not in Spemann's 

 sense. 



THE GIRDLE 



THE PECTORAL GIRDLE 



In the amphibian, Harrison ('18) showed 

 that if the limb disc (3V^ somites in diam- 

 eter) were removed the central parts of the 

 scapula would not develop. The peripheral 



& 



s.sc. s.sc. 



o 



COR. COR. 



Fig. 153. S. Sc, Suprascapula; 5c., scapula; P. 

 Cor., procoracoid; Cor., coracoid; Hum., humerus 

 (from Harrison, '18). 



parts, suprascapula, coracoid and procora- 

 coid, however, were represented by small 

 stubs of cartilage with a shape fairly char- 

 acteristic of the part which should appear 

 in the specific region occupied by the above 

 structures. (See Fig. 153.) 



This finding corroborates Braus' ('09) 

 results and indicates an early localization 

 of the parts of the girdle. If, ?.owever, the 

 animals were kept for fairly long periods 

 (ca. 100 days) the rudiments of the outlying 

 girdle parts coalesce into a single cartilag- 

 inous mass. This process was studied in de- 

 tail by Detwiler ('18), who described the 

 steps in restitution of the total structure 

 and who further circumscribed the parts 

 giving rise to the girdle. 



From his resvilts, Swett's ('23) work and 

 some unpublished experiments of my own, 

 a diagram can be constructed which shows 

 the localization of parts as we now under- 

 stand it. These are projected upon the lateral 

 flank of Amblystoma (stage 29) as shown in 

 Figure 154. 



The suprascapula develops as a good sized 

 rudiment in the absence of the limb. This 



is the most constant of the developing single 

 parts, maintaining its morphology and posi- 

 tion with a greater degree of constancy than 

 the other two outlying parts, both of which 

 tend to lie closer than normal to the center 

 of where the limb would have developed. 



The procoracoid particularly is affected 

 by limb disc removal and is frequently a 

 much smaller vestige, whereas the coracoid 

 is less affected. The suprascapular rudiment 

 develops from an area that impinges upon 

 the borders of the mesodermal somites; the 

 area indicated by hatching in Figure 154 

 possesses a greater degree of regional deter- 

 mination than is found in the other two parts 

 and is least affected. 



The chondrogenesis of the girdle seems to 

 follow a similar pattern in the Amphibia. 

 Wiedersheim (1889) found the same se- 

 quence of formation in Triturus, Siredon and 

 Salamandra that Detwiler finds in Am- 

 blystoma. Braus ('09) records just about the 

 same course of events in Bombinator. There 

 are three centers, one for the scapula, one 

 for the coracoid and one for the procoracoid. 

 There is normally no separate center for the 

 suprascapula, but in the absence of the limb 

 this does chondrify separately from the other 

 two outlying elements. 



In the normal animal the girdle and its 

 parts are readily recognizable by dissection 

 after a light staining in toto with Ehrlich's 

 haemotoxylin. Before the limb has elongated 

 and while it is still a shelf -like projection 

 from the body wall, the preparations will 

 show an aggregation of heavy mesenchyme 

 which differentiates into characteristic pro- 

 chondrin before the stage of trifurcation, 

 which indicates the appearance of the first 

 digit. The prochondrin differentiates directly 

 into chondrin, and cartilage is formed about 

 the twentieth day of larval life. At this time 

 the ulnar digit is just beginning to form. 



The scapula and the outlying parts which 

 enter into the formation of the glenoid 

 cavity wdll ossify after metamorphosis, the 

 remaining parts retaining their cartilaginous 

 character. 



During development the limb and the 

 girdle present embryonic systems in com- 

 bination in a most interesting way. Det- 

 wiler's ('18) work shows the mosaic char- 

 acter of the girdle; Harrison's ('18) resvilts 

 prove the equipotentiality of the limb. These 

 two systems differentiating in normal con- 

 tiguity produce a harmonious morphologi- 

 cal system, the appendicular skeleton in 

 which the girdle elements, anchored as they 

 are to the body wall, act as the regulators 



