FOKMATION OF GEEMINAL LAYEES IN CHELONIA. 225 



we have in the mentioned strnctiire of Trionyx tlie remnant of the 

 yolk-plug-, which appears coni^picuoiisly in the Amphibian egg. 

 Strahl describes the same structure (compare No. 13, ser. iil, figs. 

 0, 0.1, 0.2; ser. iv, figs. 0, 0.1; ser. v, figs. 0, 0.1, 0.2, 0.3; ser. vi, 

 figs. 0, 0.1; ser. vii, fig. 0.1, also No. 9, Taf. i, figs. 6, 7, 14, and 15; 

 and No. 10, figs. 2 and 3), but, so far as known to us, has never 

 explained its nature. Kupffer describes the " Zapfen " occupying the 

 horseshoe -shaped blastopore of Lacerta (No. 5, Taf. i, figs. 2 and 3, z), 

 but does not state its homology. He mentions that in Coluber 

 Aesculapii the plug is sometimes divided into two parts by a 

 median fissure (No. 5, Taf. iv, fig. 40, / and g). We have also 

 observed a similar appearance in some of the earlier embryos of 

 Trionyx, but we are satisfied that there is no true median fissure. 

 What appears to be such is the optical expression of the primitive 

 streak, along which the ectoblast is proliferating, and giving cells to 

 the mesoblast below. Even in the earliest embryos with this ap- 

 pearance it is doubtful if it ever extends to the extreme tip of the plug. 

 As far as we are aware, the only author who mentions what seems to 

 be the yolk-plug in an amniotic Vertebrate is Gasser, who observed it 

 in an abnormal fowl embryo (No. 4, Taf. x, figs. 4 — 7). The reason 

 why the yolk-plug in Trionyx is more conspicuous at this stage than 

 earlier stands, we think, in close connection with the flxct that the 

 blastopore has l)ecome a much better defined horseshoe-shaped slit. 



We return now from this long digression to the description of 

 the embryo before us. The sections behind fig. 18 show that im 

 mediately behind the yolk-plug, whicli persists distinctly in only one 

 more section after fig. 18, the ectoblast extends over the whole surface 

 as shown by the characteristic columnar cells. For a short space, 

 however, the ectoJjlast is proliferating in the median line and is 

 continuous with the mesoblast lielow. This is seen in only three 



