PLATE 3 



EXPLANATION OF FIGURES 



17 Photograph of a section through the midbrain of the same meroplast as 

 in figure 16, showing well developed dorsal aortae and the absence of a tubular 

 pharynx in a tubular head. Fusion of entoderm and ectoderm at points indi- 

 cated by .T (X 120). /, ectoderm; g, entoderm; h, dorsal aorta; i, midbrain. 



18 Photograph of one of the first available sections of the blastoderm behind 

 the incision G to // of E.xperiment Type II, no. 3, showing the freedom of the 

 pellucid area from endothelium (X 160). 



19 Photograph of a transverse section of the removed l)lastoderm of Experi- 

 ment Type I, no. 31. One of the first available sections behind the transverse 

 incisions, showing especially well the position of the longitudinalincisionsand 

 the status of the extraembryonic vascular tissue. Endth., endothelium; Pv.tnch., 

 prevascular mesenchyme. 



20 Photograph of the removal bhistoderm of Experiment Type I, no. 122, 

 showing the absence of all vascular tissue at the time of operation (i)rior to 

 somite-formation). See also figure 22. 



13(5 



