FATE OF BLASTOPORE, ETC., IX CHELONIA. 



63 



Woodcut VIII. 



nnd that the part« which are found in the sections of Series X are all 

 confined to the lower part of the sections in Series XL This shows that 

 the tail is formed l^y a new elevation or upheaval and not by the 

 tucking in of the blastoderm under something whicli previously existed. 

 If the latter process had taken place, the parts found in the sections 

 of Series X ought to be near the dorsal surface, and not left far 

 hdon\ in Series XI. A diagram will make this point clear. Let 

 Ä B in Woodcut VIII re}n-esent the original level surface of the 



blastoderm. Then the 

 tail is not formed l)y 

 tucking in as sIkjwu by 

 the line CUE, but by the 

 graduai elevation o-jven 

 by the lines FC, GC, 



Diairram to illustrate tl.e foruiation of the tail. and HC. 



I wish to add a few words in regard to the median ridge on the 

 floor of the posterior amniotic tid)e. Of the three species I haA^e 

 studied, Cleniinijs al(^ne presents this feature. In Chelo)iia and Trionija-, 

 so far at least as I liave examined them, the yolk-plug stops always 

 within reasonable distance. Ikit k seems to be a characteristic of Cleni- 

 mi/s that what appears to be the posterior prolongati(jn of the yolk-plug 

 attains such a length as that described in this Series (o). I am unable 

 to see its exact significance, but it; seems reasonable to suppose that it has 

 arisen with some relation to the yolk-phig. Mehneht ('95) has seen it 

 in Enujs lutaria tanrica (Taf. X. Fig. 26) and gives an entirely different 

 explanation of the structure. From what has been stated above it will 

 not be surprising that I find myself unai)le to accept his views.* 



D" 



* In passing, I may remark that I have never found in the species I have examined any 

 structure which I can identify with, or can consider as resembling-, Mehnert's " retroembryonal 

 Ectoderm diverticulum." 



