FATE OF BLASTOPORE, ETC., IN CHELONIA. IQl 



as descended from the secondanj merolDlastic egg by the 1(3S8 of the 

 secondanj volk-nia-ss. Hence the remnants of hoth the priinanj, and 

 the sccondm-ij, yolk-masses ought to be found in the mainmahan egg. 

 That this is actually tlie case, some of tlie recent memoirs on the 

 mammalian development make highly probable. I refer here only to 

 Van Benedex (^88), Capjus (^88), and Keibel ('93 and '95). The 

 first of these authors mentions a structure evidently comparable with 

 that which [ have called the yolk-plug in Chelonia, as the quotation 

 given at the beginning of this article (/>. 5) shows. Carius gives, in 

 Fio-. -7 of his article a section throuo-h Hexson's knob of a rabbit- 

 embryo which shows an unmistakable "yolk-plug" and is strictly 

 comparable witli Series I,/, of this article. Keibel ('95) describes a 

 peculiar cell-mass continuous with tlie epiblast at the posterior end of 

 the primitive streak in Sns. It would he rash to say that this cell-mass 

 has something to do with the yolk-remnant described in this paper, but 

 it would not be too much to say that an examination of the ])Osterior 

 end of tlie primitive streak in Sh.s in the light of the facts now 1)rought 

 out in Chelonia may result in the discovery of new relations not before 

 thought of. So much seems certain that in some respects there is 

 more agreement between Mammals and Reptiles than between Birds 

 and Reptiles. 



EXPLANATION OF DIAGRAMS MX GIVEN ON PL. X. 



In order to facilitate tlie comprehension of the views set forth 

 above, I have constructed a series of diagrams that shows the course 

 of development in Elasmobranchii and Chelonia. They are based on 

 the diagrams given by 0. Hertwig in his article " I-rmund und 

 Spina bifida " ('92) and in his Lehrbuch (4te Auß., p/). 135 d seq.). 

 These I have taken for mv basis, for, being in a much read text-book, 



