Preface. 
The Normal Plates of the rabbit were originally undertaken by me in 1896, in response to the in- 
vitation with which my friend, Professor KEiBErL, the Editor of the Series, honored me. It seemed to 
me that the rabbit offered particularly favorable opportunities for obtaining stages, which should be really 
nearly normal, i. e., representative of the median of the variations for each selected age. Accordingly I 
began collecting litters of embryos of known ages from nine to twenty-one days, the ages selected being 
always either even days or half-days. Of each age at least four litters were secured, and of some ages six 
or seven. The next step was to select for each age by careful comparison of the specimens of that age 
with one another that litter of embryos which appeared nearest central. Out of this litter three embryos 
were taken for sectioning as representing the norm for that age. In a few cases the selected embryos were 
not all from the same litter. Next the selected normal or median embryos of all the ages were compared 
with one another to make sure that they formed a good progressive series. A typical embryo of each set 
of three was drawn and thus the series of figures on the plates was prepared. As will be seen the method 
worked satisfactorily on the whole, though the “normal” embryos of twelve and one half and of thirteen days 
do not fit perfectly into the series figured. 
The three selected embryos of each stage were sectioned, one in the transverse, one in the sagittal 
and one in the frontal plane. The three series of sections in each case have been added to the Harvard 
Embryological Collection, where they will be always accessible to competent investigators. The data as 
to the development of the embryos have been obtained from the study of these series, and similar ones of 
younger stages. Study soon showed that the three “normal” embryos agree very closely with one another in 
the details of their development, so that as a rule a correct collective statement as to the condition of each 
organ could be drawn up with little difficulty so as to be applicable to all three embryos. Exceptions to 
this rule are not very frequent, and all the important ones observed are noted in the tables. 
Of the older stages (nine to twenty days), it may be claimed, I think, that we really have considered 
“normal” embryos. 
With the younger stages a like success was not attained, hence in regard to these it must be pointed 
out that the descriptions refer to individual embryos as has been the custom in the previous numbers, 
I—IV, of the “Normentafeln”. 
After I had prepared a good deal of material, in fact most of that used for this work, and had made 
some progress with the collection of titles for the Bibliography, it became clear to me that I could hardly 
hope to complete the work in a reasonable time, so great had the pressure of my avocations become. 
Normentafeln zur Entwicklungsgeschichte der Wirbelthiere. V. I 
