370 EYCLESHYMER. [Vol. X. 



Marshall ('93) confirms the observations of Robinson and 

 Assheton, stating that " the reduction is effected, not by a con- 

 traction of the whole circumference of the blastopore, but by a 

 folding together, or concrescence, of its lips in the median 

 plane, beginning at the lower or ventral margin and proceeding 

 upwards towards the dorsal margin." 



Such a procedure I have never observed in Amblystoma or 

 Rana palustris, although Jordan ('93) found it occurring in the 

 Newt. 



Before the neural folds are well defined (Fig. 5) the neural 

 groove {n.g) appears; in some cases it runs forward as a continua- 

 tion of the slit-like blastopore, while in others it is not continu- 

 ous, but an independent groove (Fig. 5). A second groove is 

 often observed lying between the posterior end of the neural 

 groove and the blastopore; often it arises at, and is continuous 

 with, the dorsal lip of the blastopore. 



2. The Origin of the Mesoblast and Chorda. 



The study of the early mesoderm may best be introduced by 

 examining a meridional section (PI. XXI, Fig. i) of such an Q.g^ 

 as that shown in PI. XX, Fig. 7. 



The segmentation cavity (s.c^ in most cases shows a radial 

 symmetry, although occasionally its bilaterality is strongly 

 marked, the roof being composed of from two to three layers 

 of cells. The slight infolding shown in surface views is here 

 observed to occur along a line where the entoblastic cells shade 

 off into the smaller epiblast cells. I shall henceforth speak of 

 this infolded blastodermic rim ig.r) as the germ-ring, since I 

 believe it to be homologous with a like thickening in the blasto- 

 derm of Teleosts and Elasmobranchs to which this term is 

 generally applied. Just within and above the line of invagina- 

 tion there is a group of cells {m) which are larger than the 

 adjoining ectoblastic cells, and possess more pigment than the 

 entoblastic cells. These cells, extending around the entire &g^t 

 form a ring which later gives rise to the mesoblast. 



Fig. 2 represents a vertical section along the line xy of 

 PI. XX, Fig. 2. The roof of the segmentation cavity is some- 

 what thinner than in the preceding stage, being in some por- 



