342 JORDAN. [Vol. VIII. 



the area pellucida of the chick blastoderm, has since come 

 to obtain a wider significance. Homologous areas have been 

 alleged to exist in the early stages of other vertebrates, and 

 some of these cases have been looked upon as throwing light 

 on the meaning of this primitive and persistent structure. 

 The macroscopic appearance of a "streak" is simply the out- 

 ward expression of an inner thickening of the embryonic area, 

 due to the fusion of the tJirce primary gei'in-laycrs. This fusion 

 must be regarded as the most characteristic mark of the prim- 

 itive streak and can evidently be discovered only from sections, 

 so that observations of a ''primitive streak" based on surface 

 views alone are of doubtful value. Taking as a criterion of 

 the primitive streak, then, the fusion of germ-layers, what is 

 the hidden significance of this early union of the three layers 

 in a linear mass or streak, tJic long axis of which invariably 

 coincides with the long axis of the embryo ? 



It is a fact of no small importance that the rim of the 

 blastopore is the region where the fusion of layers first makes 

 its appearance in many vertebrate embryos. This fact alone 

 tempts one to believe that the primitive streak is the result 

 of the fusion of the lips of the blastopore in the mid-line. 

 The comparison of the line of fusion of the lips of the Elas- 

 mobranch blastopore with the primitive streak of the Amniota 

 was made long ago by Balfour, who, however, expressly dis- 

 claimed any implication of homology. 



Assuming that in the newt the primitive streak is the linear 

 mass of fused germ-layers formed by the bringing together of 

 the blastopore lips, let us determine its extent and character 

 without, for the instant, any reference to possible homologies. 

 Figs. 52 and 55 represent median sagittal sections through 

 early blastopore stages. The only region of fusion is that 

 immediately around the yolk-plug. This fused area is seen in 

 these sections to be slightly greater at the posterior (ventral) 

 lip, but this fact does not come out so clearly as in recon- 

 structions. The lateral lips of the blastopore begin to approx- 

 imate shortly after the stage shown in Fig. 55, and a horizontal 

 section through Fig. 40 in the line 63-63 (Fig. 63) shows the 

 lips almost touching. A median sagittal section through the next 



