THK Dl!;VELOPMENT OF LEPIDOSIIIEN PAKADOXA. 35 



Passing on to Lepidosiren, the difference between it 

 and Petromyzon is seen to be quite insignificant, consisting, 

 in fact, only of difference in relative dimensions. 



Finally, the condition of the mesoblast in one of the higher 

 Vertebrata, as indicated in fig. D, seems to me to hang on 

 equally well to the earlier members of the series. What dif- 

 ference there is is merely difference in shape and relative 

 size. I hold, then, that in the series of Yertebrata there 

 exist passing phases in the development of the mesoblast 

 which may be readily linked on to one another, and that the 

 existence of these phases may be accounted for by regarding 

 them as reminiscences of phylogenetic stages in the modifica- 

 tion of the process of mesoblast development.^ 



Conclusion. — In general the phenomena described in 

 this paper fully bear out what I referred to in my earlier 

 communication — the extreme resemblance with corresponding 

 features in the Urodela. As regards external features 

 during the earlier periods of development this likeness is 

 perhaps slightly less marked in Lepidosiren than in Pro- 

 top ter us, but as regards internal features of segmentation 

 and gastrulation the most remarkable resemblance is seen. 



The resemblance with Petrom^^zon is equally striking, 

 and that with Ganoids only slightly less so. I do not feel 

 it necessary to go into detail in this matter; it will only 

 be necessary for the reader to turn to such figures as 

 Houssay's 2 pi. xi, fig. 26 (transverse section of Axolotl, 

 showing early stage of mesoblast) ; Calberla's ^ fig. 7 

 (similar section through Petromyzon); Eycleshymer's* 

 pi. XX, fig. 8 (external view during early invagination of 



1 It will be noticed that, on the above hypothesis, the growth of the meso- 

 blast at its outer side by continued delamination from the hypoblast would 

 correspond to a continued deepening of the groove between mesoblastic and 

 chordal rudiments of Amphioxus, and is therefore easily understood. 

 Were Hertwig's scheme the true one this growth of the mesoblast would be 

 quite incomprehensible. 



- 'Arch. Zool. exp.,' 2nd serie, tome viii. 



^ ' Morph. lahrb.' iii, Taf. xii. 



* ' J. Morphol.,' vol. X. 



