FORMATION OF GERMINAL LAYERS IN CHELONIA. 29 



Rusconi found in the Amphibian eggs. Allowing for the 

 differences between a holoblastic and a meroblastic egg, the 

 relations in the two figures are almost exactly alike, part for 

 part. If the slits between the plug and the lateral lips of the 

 blastopore extended in our figure a little more into the midst 

 of each mesoblastic mass the resemblance would be complete ; 

 but even for the Amphibian eggs the slits do not always extend 

 as far as represented in fig. 9, as Hertwig himself mentions 

 (No. 6, p. 14). At any rate, in each case there is an axial 

 mass of cells, (1) into which the ectoblast turns down at the 

 lateral lips of the blastopore, (2) in which the entoblast is not 

 to be distinguished, (3) from which the mesoblastic masses 

 start away toward each side, and (4) which sends a plug upwards 

 between the lips of the blastopore. If we compare the longi- 

 tudinal section of the plug in Trionyx (fig. 24, yk. p.) with the 

 sagittal section of the Amphibian yolk-plug (Hertwig, No. 6, 

 Taf. ii, fig. 4), we see again that the relations of different parts 

 are alike. It is true that the plug in Trionyx is not bounded 

 posteriorly by a groove, and passes directly into the ectoblast 

 of the primitive streak, but when we consider that the plug in 

 Trionyx is only rudimentary this is not to be wondered at, and 

 is of little significance. 



We think we are justified, on these grounds, in concluding 

 that we have in the mentioned structure of Trionyx the rem- 

 nant of the yolk-plug, which appears conspicuously in the 

 Amphibian egg. Strahl describes the same structure (compare 

 No. 13, ser. iii, 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 bv a 

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

 also observed a similar appearance in some of the earlier em- 

 bryos of Trionyx, but we are satisfied that there is no true 



