22 



medial and caudal hoiuidaries of the thinned cenlrnni, a small trace 

 of bone-tissue is seen, a( a place almost corresponding to the 

 crossing-point of the definite episternum. The blastematic medial ends 

 of the claviculae are no longer sharply defined ; and there is no 

 connection yet between the two by way of the medial line. 



Embryo liacerta ag. 0. (N. T. about 32). The changes in the 

 primary girdle are confined here to the non-cartilaginous portions. 

 At the place of the incisnra scapnlo-procoracoidea and of the fenestra 

 principalis the thinning of the tissue is fairly complete ; the blastema 

 once present has become nearly a membrane of connective tissue. 

 In the cranial border of the membrana scapnlo-procoracoidea a 

 thicker strand is distinguishable, in which a few praechondrium 

 insulae are lying, as the remains of a entiiely praecliondral bridging. 

 In the retro-clavicular blastema the central thinning has |iroceeded 

 further. Fig. 4 shows five partial illustrations of sections from this 

 series. Each has been drawn exactly to the medial plane. There 

 was still loose mesencliyme between the right and left claviculae. 

 In Fig. a the bony clavicula has l)een taken for the greater part 

 lengthwise. In Fig. b only the thickened medial end of the clavicula 

 is to be seen. Lateral to it comes a thinner blastematic region 

 (thinned centre), still more laterally the cut caudal border. In Fig. c 

 only the thinned centre with the caudal surrounding border medial 

 and lateral to it has been re|)roduced. Of the clavicular bone no 

 more traces are to be seen. Figs. </ and e have been chosen caudal 

 to the thinned centre. Fig. e, the most caudal, shows the last vestige 

 of the retroclavicular blastema. 



Embryo Lacerta ag. M. (N.T. about 33). In this one the thinning 

 into a connective tissue membrane has been fully accomplished at 

 the place of the incisnra scapulo-procoracoidea and of the fenestra 

 principalis. Apart from a oraechondral insula, the incisnra bridging 

 consists of a strand connective tissue, ligament. The central thinning 

 of the blastema lying caudal to the bony clavicula has here, too, 

 practically led to the formation of a connective-tissuemembrane. The 

 bony episternum has grown in size. There is thus now one connected 

 complex present, consisting of a thin bony episternal transverse bar 

 (situated in the caudal boundary), from which a ihin blastema 

 bundle can be traced to a point ventral of the equal-sided sternal 

 band and in which a still much thinner fragment of bone, (even 

 broken perhaps locally) is found. Thus, for the first time, in this 

 embryo a small piece of the episternum is met with ventral from 

 the sternal formation. From the transverse bar a blastema bundle 

 (likewise caudal border) runs in a lateral and cranial direction and 



