204 FORMATION OF THE LAYERS. 



hypoblast. In the region of the embryo the mesoblast plate is 

 stated by Kupffer and Benecke to be continuous across the 

 middle line, but this appears very improbable. In a slightly 

 later stage the medullary plate becomes marked by a shallow 

 groove, and the mesoblast of the embryo is then undoubtedly 

 constituted of two lateral plates, one on each side of the median 

 line. In the median line the notochord arises as a ridge-like 

 thickening of the hypoblast, which is continued posteriorly into 

 the front wall of the passage mentioned above. 



The notochord does not long remain attached to the hypo- 

 blast, and the separation between the two is already effected for 

 the greater part of the length of the embryo by the stage repre- 

 sented in fig. 129. Fig. 126 represents a series of sections 

 through this embryo. 



In a section (A) through the trunk of the embryo a short 

 way in front of the primitive streak, there is a medullary plate 

 with a shallow groove (mg), well-developed mesoblastic plates 

 (mep), already divided into somatic and splanchnic layers, and a 

 completely formed notochord independent of the hypoblast (fiy). 

 In the next section (B), taken just in front of the primitive 

 streak, the notochord is attached to the hypoblast, and the 

 medullary groove is deeper ; while in the section following (C), 

 which passes through the front border of the primitive streak, 



FIG. 127. DIAGRAMMATIC LONGITUDINAL SECTION OF AN EMBRYO OF LACERTA. 

 //. body cavity; am. amnion; ne. neurenteric canal; ch. notochord; hy. hypo- 

 blast ; ep. epiblast of the medullary plate ; pr. primitive streak. In the primitive 

 streak all the layers are partially fused. 



the notochord and hypoblast have become fused with the 

 epiblast. The section behind (D) shews the neurenteric passage 

 leading through the floor of the medullary groove and through 

 the hypoblast (ne). On the right side the mesoblastic plate has 

 become continuous with the walls of the passage. The last 

 section (E) passes through the front part of the primitive streak 



