DESCRIPTION OP PLATES Vl-IX-continued. 



primitive streak the blastoderm is perforated ; a few mesoblast cells are present 

 anterior to the perforation. 



Fig. 32. — Transverse section through the region, anterior to the primitive 

 streak, of an embryonic area similar to that drawn in fig. 12. The single row 

 of hypoblast seen in fig. 30 is here giving rise to hypoblastic mesoblast. 



Figs. 33, 34, and 35. — Transverse sections through the blastoderm drawn 

 in fig. 12. 



Fig. 33 is a section through the head of the primitive streak. The layer of 

 cells below the epiblast is nowhere differentiated into hypoblast and mesoblast, 

 though a few cells (i^.) ex.hibit a tendency to flatten out and become sepa- 

 rated from the rest. A keel of epiblast {ep. k.) is forced into the underlying 

 hypoblastic mesoblast in the middle line. 



A few sections further back (fig. 34) the anterior region of the primitive 

 groove is cut through. The hypoblast, epiblast, and mesoblast are united in 

 the middle line, while laterally they are distinct from one another. The 

 mesoblast does not extend beyond the embryonic area in this region. 



Fig. 35 is a section through the middle of the primitive streak where the 

 epiblast and mesoblast are united in the middle line, and the hypoblast is free 

 throughout. The mesoblast here extends beyond the embryonic area. 



Fig. 36. — A transverse section through the hind knob of the primitive 

 streak of an embryonic area very similar to that seen in fig. 12. The epiblast, 

 mesoblast, and hypoblast are throughout the entire breadth of the area indis- 

 tinguishable from one another ; the mesoblast, however, extends beyond the 

 area, and they are there distinct. 



Figs. 37 to 41. — Transverse sections through the neurenteric canal of an 

 embryo slightly older than the one represented in fig. 13. The dorsal opening 

 of the canal is seen in fig. 37 ; it passes thence through the meoblast (fig. 38) 

 to the hypoblast, along which it extends for some distance (figs. 39, 40), and 

 eventually opens ventrally in fig. 41. The three layers are united together 

 at the front end of the primitive streak (fig. 37). Anteriorly the hypoblast is 

 first separated as a thin layer (fig. 38), and then becoming much thickened 

 contains the neurenteric canal (figs. 39 — 41). Where the neurenteric canal 

 enters the mesoblast the latter is united with both epiblast and lateral meso- 

 blast, but freed from hypoblast. When the canal enters the hypoblast the 

 lateral mesoblast becomes at first distinct, but afterwards is continuous with 

 the axial mass of cells. Compare Fig. 50. 



Fig. 42. — Transverse section in the anterior region of the medullary groove 

 of the same embryo, showing the thickened axial hypoblast to which the lateral 

 mesoblast and hypoblast is joined. 



