62 MR W. E. AGAR ON THE DEVELOPMENT OF THE 



from their bases. The arch is between the third and fourth metotic myomeres 

 (Z and A). 



The quadrate is from the first continuous with the trabecula. There is no 

 hyomandibular. 



There appears to be a vestige of a palato-pterygoid cartilage. 



The parasphenoid develops from paired rudiments, separated by the hypophysis. 



The internasal septum is formed by the fusion of the anterior ends of the trabeculse. 

 The nasal capsule is formed by outgrowths from this septum, with fusion with the 

 anterior upper labial cartilage. 



There are two upper labial cartilages, anterior and posterior. 



The cartilage in connection with the interoperculum appears to be a hyoidean ray 

 (Huxley, Bridge). 



The dermal ectethmoid develops in connection with the temporal muscle. 



The cartilaginous cranial roof in the auditory and occipital region is confined to the 

 supra-occipital cartilage in Lepidosiren, while there is in addition a tectum synoticum 

 in Protopterus. 



In the successive stages from the embryo to the adult the chondrocranium shows a 

 steady relative increase in completeness. 



EXPLANATION OF THE PLATES. 



Plate I. 



Camera drawings of median sagittal sections illustrating the development of the front end of the 

 notochord in Lepidosiren. 



Fig. 



1. 



Stage 25. Zeiss D. 2oc. 



Fig. 



2. 



Stage 26. Zeiss A. 4oc. 



Fig. 



3. 



Stage 31. Zeiss A. 4oc. 



Fig. 



4. 



Stage circ. 32. Zeiss A. 4oc. 



Fig. 



5. 



Stage 38. Zeiss A. 4oc. 



Fig. 



6. 



Stage - 35. 3 mm. imm. 2oc. 



Plate II. 



Fig. 7. Protopterus, stage circ. 31. Reconstruction (by Graham Kerr's method) from horizontal sections. 



Fig. 8. Lepidosiren, stage 34. Reconstruction from horizontal sections. X, Y, Z, etc., are the 

 myomeres corresponding to the nerves x, y, z, etc. The bones are shown in outline on the transparency. 



Fig. 9. Lepidosiren, stage 36 + . Reconstruction from horizontal sections. 



Fig. 10. Horizontal section through the ventral end of the quadrate of Protopterus, stage circ. 31, to 

 show the connective tissue strand representing the vestige of the palato-pterygoid cartilage. Zeiss A. 4oc. 

 Camera lucida. 



Fig. 11. Similar section {Protopterus), stage 34. Zeiss A. 4oc. Camera lucida. 



Fig. 12. Transverse section through the thalamencephalon in Lepidosiren to show the back-growth of 

 the palato-pterygoid bone along the inner side of the connective tissue strand. Stage 34. Slightly higher 

 magnification than figs. 10 and 11. Camera lucida. 





