eral of these disappear through fusion in the course of devel- liooi 

 opment. Allowing for the possibility that these observers 

 have counted in an occipital segment^ there wonld be in 

 embryos of this size at least thirty-seven trunk segments, 

 which would correspond to thirty-six vertebrae. Keibel finds 

 in an embryo of 8 mm. thirty-five trunk segments, together 

 with a mass of unsegmented mesoderm, equaling two seg- 

 ments in length. Eeckoning this as two instead of one seg- 

 ment, as Keibel does, we have again thirty-seven segments, 

 corresponding to thirty-six vertebrae. 



The following is an attempt to tabulate the number of 

 segments found in embryos varying in length from 7.5 to 

 21.5 mm. With the exception of the last column the data 

 are as recorded by the observers themselves. In the last 

 column the number of vertebrae is given which would corre- 

 spond to the total number of segments after certain changes 

 have been made, such as deduction of occipital segments or 

 addition of unsegmented mesoderm, which seemed justified by 

 the descriptions of the authors. 



[ion 



Corresponding 



Length Segments Sninal number of 



Observer. of embryo in mes- ffano-lia Vertebrae, vertebne after 

 in mm. oderm. ^ "= ' allowing for 



corrections. 



His 



7.5 



35 





34 



34 



Keibel 



8.0* 



35 ^ unsegmented 

 mesoderm. 







36t 



Fol 



8.0-9.0 



38 





36 



36 



Phisalix . . . . 



10.0 



38 



36 





36 



Keibel 



11.5* 





34 



35 



36t 



mesoderm. 







Fol 



12.0 







36 



36 



Harrison . . . 



14.0 





33 



36 



36 



Harrison . . . 



16 





32 



37 



37 



His 



16.0 







34 



34 



Rosenberg . . 



16.5 







33 



33 



Fol 



19.0 







34 



34 



Rosenberg . . 



19.0 







35 



35 



His 



31.5 







34 



34 



* Neck-breech measurement. 



t Counting the terminal mesoderm as equivalent to two segments. 



(15) 



