90 



J. Thos. Patterson. 



Measurements taken on the living eggs also can be interpreted 

 in support of this view ; for such data show that previous to and 

 following gastrulation the blastoderm is approximately circular, while 

 during gastrulation the antero-posterior diameter is always shorter 

 than that of the transverse (Table I). This is what we should 

 expect in case the margin actually involuted. 



Owing to individual variation in the size of different blastoderms 

 at the same stage of development, it is impossible to determine, from 

 the above table, whether the antero-posterior diameter is actually 

 shorter after the beginning of gastrulation than just preceding the 



TABLE II. 



same, or whether it is only relatively shorter in comparison with the 

 transverse diameter. If it can be shown that the former alternative 

 is the true one, then the evidence for a "plain rolling under" of the 

 margin will be well nigh conclusive. This I have been able to do 

 by studying the living egg and taking measurements of the same 

 blastoderm at different periods of its development. In the above 

 table are given the data from such measurements taken on two eggs. 

 Finally, the above interpretatioTi for the origin of the entoderm 

 is in harmony with the views of a large majority of the investi- 

 gators who have worked on other groups of vertebrates. It is with 

 the fish, however, that the most interesting and instructive com- 

 parisons are to be drawn. The large size of the selachian ovum, 



