Scope of the Paper. S 



Emys europsea, Lacerta, and Coluber asclepii, and denoted as 

 "Zapfen"; which Strahl ('83 and '84) has figured for Lacerta 

 agilis and named " Caudalknoten " ; and which has been repeatedly 

 figured by Mehnert ('92) for Emys lutaria taurica, by Will 

 ('92'' and '93) for Platydactylus facetanus and Cistudo lutaria, 

 by Mitsukuri ('93) for Chelonia caouana, Trionyx Japonicus, 

 and Clemmys Japonica, and by van Beneden ('88, p. 711) for 

 Mammalia. The last four writers have made this structure 

 homologous with the yolk plug of Amphibia. This structure I 

 noted constantly in older shields, but the homology, as made out 

 by the above mentioned observers, I cannot accept. 



My sections moreover compel me, so far as concerns the 

 species I have studied, to disagree with Will in regard to his 

 theory of an uncovered entodermic streak. Furthermore, I agree 

 with Mitsukuri in so far as he is unable to discern that sharp 

 boundary between ectoderm and entoderm which Will figures for 

 the lateral and posterior margins of the streak. 



Sections of embryos which show conditions later than the early 

 formation of the notochord, gastral mesoderm, and backward 

 bending of the lateral portions of the blastopore, will not be 

 described. 



This paper will be confined to the conditions found in a small 

 collection of twenty-six embryonic shields, including (i) stages 

 in which the blastoporic opening is a crescentic slit concave in 

 front, (2) those in which the slit becomes a transverse opening, 

 and still later (3) those in which the crescent is concave behind. 

 These stages furnish proper material for the study of the questions 

 concerning the yolk plug and the entodermic streak. 



III. Description of the Notochordal Invagination. 



I. Chelydra serpentina. — Only two series of sections were 

 obtained of embryos with a developing notochordal canal intact. 

 Nor can these two series be considered to show absolutely normal 

 conditions, for the embryos from which they were taken were a 

 pair of twins, or doublets, upon the same embryonic disk. Plate 

 II. Fig. 8, and Diagram I., represent the dorsal surface view of 

 these twins. The blastoporic opening of the embryo a* which 



* The letters a and j3 refer to Diagram I. (page 6). In Figure 8 these 

 letters have been accidentally interchanged. 



