ORIGIN OF VASCULAR TISSUES 69 



which severed the anterior and posterior portions of the embry- 

 onic body, and which also was the transverse Hne of separation 

 of the blastoderm. In this experiment all the blastoderm an- 

 terior to the line G-H was left for further incubation, while that 

 posterior to it was removed. Figures 16, 17, and 18 show the 

 conditions as seen in sections of the meroplast and its blasto- 

 denn-control. Figure 16 is a cross-section through the anterior 

 portion of the fore-brain. The cut edges of the entodermal and 

 ectodermal layers of the proamnion have fused, forming pos- 

 teriorly a bUnd sac into which there seems to have grown from 

 the anteriorly lying coelom, a sac which likewise ends blindly 

 within the posterior region of the proamniotic sac. Formerly 

 I regarded the presence of this coelomic sac as abnormal. It is 

 probably a normal structure, so far as the presence of coelom 

 in the proamnion at this stage is concerned. It will be seen that 

 the cut edges of ectodenn and entoderm of the outlying blasto- 

 derm have fused. The only possible path by which yolk-sac 

 vessels could have reached the embryonic axis would have been 

 by way of the proamniotic sac. It will be noted that the latter 

 is devoid of blood-vessels. 



Figure 17 is a section through the mid-brain region of this 

 same meroplast. Its interpretation will be facilitated by refer- 

 ence again to figure 1. Since the incision G-H was made con- 

 siderably behind the anterior intestinal portal, the axial tissue 

 included between the longitudinal incisions and lying between 

 the anterior intestinal portal and the incision G-H was neces- 

 sarily composed of three layers at the time of operation. Dor- 

 sally there was ectoderm, ventrally entoderm, and between the 

 two mesoderm was present. The cut edges of ectoderm and 

 entoderm have fused, and the dorso-ventral expanse of the 

 contained tissue has rounded the axial portion into an almost 

 cylindrical shape. Thus the meroplast has the superficial ap- 

 pearance of a tubular head-portion of the body axis as one 

 finds in the fore-brain region projecting over the blastoderm. 

 Since this section lies 'behind the anterior intestinal portal, there 

 is no ventrally lying proamniotic sac. Although the body ap- 

 pears to be tubular, it contains no alimentary tube. The ven- 



