DEVELOPMENT OF THE AMERICAN ALUGATuR — REESE 19 



with certainty the origin of the notochord, owing to the close fusion 

 of all three germ layers. Between the wall of the enteron and the 

 lower side of the ectoderm is a considerable mass of mesoderm 

 (incs), which here consists of more scattered and angular cells than 

 in the preceding section. 



Figure 8j shows the appearance of a section through the meso- 

 blastic somites, in one of which a small myocoel (myc) is seen. As 

 is seen by the size of the figure, which is drawn under the same 

 magnification as were all the sections of the series, the embryo in 

 this region is much smaller in section than it is toward either end, 

 especially toward the anterior end. The medullary groove (wig) is 

 still more shallow than in the more anterior sections, and the ecto- 

 derm (cc), with which its folds are continuous laterally, is here 

 nearly horizontal. The mesoblast (mes) is of a more compact na- 

 ture than in the preceding section and shows little or no sign of 

 cleavage, although a distinct myocoel may be seen and cleavage is 

 well marked in sections between this one and the preceding. 



The notochord ( nt) has about the same appearance as in the pre- 

 ceding section, but is more distinctly separated from the surrounding 

 cells. 



Figure 8/ is through the posterior end of the embryo : it shows the 

 relation of parts in the region of the primitive streak. Although not 

 visible in surface views, and hence not represented in figure 8, the 

 medullary groove is continued without any line of demarcation into 

 the primitive groove, and the medullary folds into the edges of the 

 primitive streak, so that it is impossible to set any definite boundaries 

 between these structures unless the dorsal opening of the blastopore 

 be taken as the point of division. The medullary groove {wig), if 

 it be here so called, is proportionately more shallow than in the pre- 

 ceding figure and is actually much wider. The section passes behind 

 the posterior end of the notochord, so that structure is not seen. 

 Though not so well indicated as might be desired in the figure, the 

 three germ layers are here indistinguishable in the middle line, and 

 in the center of this mass of cells the blastopore (blp) or neurenteric 

 canal may be seen as a small vertical slit. As will be more fully 

 described in the following stage, this canal opens dorsally a few sec- 

 tions posterior to the one under discussion and ventrally a few sec- 

 tions farther toward the head. 



In all the sections of this stage the ectoderm and entoderm are 

 fairly thick in the region of the embryo proper, but become thinner 

 iintil reduced to a mere membrane as we pass to more distal regions. 

 Both layer? are composed of loosely arranged cells, with scattered 



