Introduction. 



In selecting the various "stages" which were to form the basis of my work upon the development 

 of Lepidosiren and Frotopterus I realized from the beginning the importance of making my series of "stages" 

 agree as closely as possible with the series of stages defined by Semon in his classical work on Ceratodus. 

 In would clearly have greatly facilitated the comparison of data obtained from the investigation of the 

 development of the Monopneumona and Dipneumona and therefore have added to the value of these data 

 if the developmental history could have been divided into exactiy corresponding stages. A little investigation 

 was sufficient to make clear the impossibility of arranging any such corresponding series of stages and 

 I was therefore compelled to make my selection of stages of Lepidosiren quite independent of those selected 

 by Semon for Ceratodus. In defining the stages of Frotopterus I have endeavoured to make them correspond 

 in number with those oi Lepidosiren but even here it will be seen that the agreement between the stages of 

 Lepidosiren and Frotopterus, although the two genera are so closely allied, is only of a comparatively rough 

 kind, and I have had to make the descriptive tables quite independent. The figures reproduced in the 

 three plates cover the developmental history of the two genera fairly completely up to stage 36, except 

 that in the case of Frotopterus the early stages of segmentation have not yet been observed. I have not 

 thought it advisable to include figures of stages subsequent to 36, when the adult form is being assumed, 

 as their inclusion in the plates available would have necessitated the reduction in size of the earlier and 

 more important figures. 



For the figures of the various stages I am indebted to the high artistic skill combined with con- 

 scientious care of Mr. A. K. Maxwell. They have been drawn throughout under my close supervision 

 and I can vouch for their accuracy. The necessary section cutting has been carried out with his usual 

 skill and care by Mr. P. Jamieson. 



The data given in the Tables are to be taken as referring to the specimens figured. In most cases 

 several, in some cases numerous, specimens belonging to the same stage according to their external features 

 have been investigated by sections and in the few cases where it has seemed necessary to include data 

 derived from such other specimens such data are enclosed between brackets. Such necessity has arisen 

 in various cases where particular points can be determined with greater accuracy in sections cut in different 

 planes from those in which the specimen figured had been cut or prepared by ditferent embedding or 

 staining methods. 



Seeing that the present part of the "Normentafeln" deals with forms so little familiär to embryologists 

 generally I have thought it advisable to preface the Tables with a short sketch of the general development 

 of the two forms in question. 



Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. X. 



