632 MR W. E. AGAR ON DEVELOPMENT OF ANTERIOR MESODERM, AND 



Stage. 



Lepidosi 



-en. 



Protopterus. 













Pelvic Fiu, Post-axial. 



Cloacal Aperture. 



Pelvic Fin, Post-axial. 



Cloacal Aperture. 





Myotome. 



Myotome. 



Myotome. 



Myotome. 



31 



56 



58 







31 + 



52-53 



54 







31 + 



52-53 



54 







32 



55 



57 



37 



40 



34 



54 



56 



36 



38 



34 



52 (pelvic plexus 

 NN45-53 + 154) 



54-55 







circ. 35 



53 



56 







36 



54 



57-58 



36 



39 



38 



56 (pelvic plexus 

 NN47-53 + ?54) 



58 







6 3 cm. 







35-36 



37 



6*9 cm. 







34-35 



36-37 



7'3 cm. 



. . . 





35 



37 



7-4 cm. 



♦ . . 





34 



36 



7*5 cm. 







35 



37 



7-8 cm. 







34-35 



36-37 



The total number of post-occipital myotomes in Lepidosiren is about 110, in Protopterus about 65. 

 In Ceratodus the pelvic fin is under spinal ganglia 28-30 (Semon). 



Examining this table we find that the variation in the position of the pelvic fin is 

 quite noticeable, but in Lepidosiren it is neither further forward nor backward on the 

 average in older than in younger individuals. In Protopterus the table gives a slightly 

 further forward position of the fin in older than in younger individuals. In this genus, 

 however, the number of young specimens examined is not sufficient to draw safe con- 

 clusions from. Any variation in the position of the fin is closely associated with a 

 corresponding variation in the position of cloacal aperture in both genera. The pelvic 

 girdle maintains a nearly constant distance in front of this opening. Thus the position 

 of the pelvic fin appears to be determined by the position of the earlier appearing 

 cloaca, and the variations now observable in its position are not of the same nature as 

 those variations which resulted in the backward migration of the fin relative to the 

 cloaca which has to be assumed on the Gegenbaur theory. The variations in the 

 position of the cloaca are not due to a "migration" of this structure relatively to the 

 myotomes either, but to an increase or decrease in the total number of myotomes in the 

 body. This is made apparent by the comparison of the two genera, which shows that 

 in Lepidosiren the position of the cloacal aperture is about M56, the total number of 

 post-occipital myotomes being about 110, and in Protopterus it is opposite M38 (about), 

 corresponding with the shorter length of about 65 myotomes. 



On the other hand, there is considerable evidence that the fin has undergone a real 

 migration in a backward direction in the past. For one thing, we find it at the hind 

 end of its innervation region. Then the history of the ectodermal rudiment points in 

 the same direction. It has just been made obvious that the position of the fin in 



