26 



parts in the primaiy girdle have given place to tliin membranes of 

 connective tissne, or, in other words, the fenestration is complete. 

 The lig. scapulo-piocoracoidenra contains praechondrium which is 

 connected with the cartilage of the girdle only by ligament The 

 bony tissue in the clavicular process has increased in extent. 



In the remaining older embryos there is but little that is new to 

 be remarked about the scapulo-coracoideum, (increase in size and 

 commencement of calcilication). The further development of the 

 clavicular foimation could not be traced. In the older embryos the 

 latter appears in the ventral body-wall, and as it is but thinly 

 covered with the skin, it is hardly possible in the frontally cut 

 series to detine the cell-thickening under the almost tangentially 

 cut breast-skin from the blastemic clavicular formation. For the 

 same reason the development of the episternum could not be traced 

 in detail. In the oldest series a paired bony episternal formation 

 was present. (Gongylus oc. E. and L.). Tlie episternal formation of 

 one half of the body has been demonstrated by me elsewhere. (Fig. 6) '). 



Ptychozoon homalochephalum. Embryo A is still very young, the 

 diaphysis humeri contains no cartilage as yet. The shoulder-girdle 

 formation is continuous with the humerus formation. The blastematic 

 scapulo-coracoideum is still rather vaguely outlined. The mesenchyme 

 thickening, of which it is formed, is quite homogeneous. Nothing is 

 to be seen yet of the fenestra principalis which occurs in the adult 

 scapulo-coracoid ; nor of the incisura scapulo procoracoidea. The bony 

 clavicle, or even the blastematic formation of it, is still lacking. 



Ptychozoon embryo B. Round the diaphysis-humeri lies a covering 

 of perichondral bone. The primary shoulder-girdle shows cartilage. 

 The more cranial portions are still praechondral (epicoracoid !). The 

 fenestration of the tirst homogeneous compact coracoideum is already 

 fairly complete. So the conditions correspond completely to those 

 found in Lacerta J. and Gongylus D — E. 



From the cranial border of the scapulo-coracoideum the bony 

 clavicula proceeds, connected with the scapula by syndesmosis. 

 Joined to the clavicula, just as in Lacerta, is a retroclavicular 

 blastema. Of the episternum no traces of bone are to be found yet. 



The examination of the embryos of Gongylus and Ptycliozoon has 

 thus led to the c.ontirmation of most of the facts observed in Lacerta, 

 namely the origin of the fenestra |)rincipalis and of the incisura 

 scapulo-coracoidea by reduction of parts of an originally compact 

 primary shoulder-girdle and also the primary connection of the 



') Gh. van Gelderen, Proceedings. Kon. Acad, v Wetenscli. Vol. XXIV, 1922. 



