216 EDWARD A. BOYDEN 



schwinden), Remak, following Reichert, believed that the new 

 elements did not displace the original wall but were merely in- 

 corporated into it as they grew in. ("Die sekundare Bauchwand 

 entsteht aus der Verschmelzung der unspriinglichen Bauchwand 

 mit der hervorwachsenden Entwickelungsproducten der Urwir- 

 bel.")- Although my own observations have resulted in the find- 

 ing of new manifestations of a migration of tissues in the body- 

 wall it is still very difficult to analyze the nature of the move- 

 ments. It may be that at a certain stage in the development of 

 the wall that part of the membrana reuniens which lies over the 

 breast is being pushed in toward the median line by the faster 

 growing lateral parts and reduced to a narrow zone, — the slack, 

 so to speak being taken up by the increasing thickness of the 

 membrana and by the protrusion of certain external and internal 

 ridges. Or it may be that this early shifting of structures is 

 more in the nature of a preliminary growth-wave which is passing 

 from vascular to non-vascular territory. 



The difficulty of analyzing these changes, however, does not 

 derogate from the conclusions which we may now form concern- 

 ing the relation of the pectoral ridges and the branchial filaments, 

 — to ascertain which this study of the pectoral region was first 

 undertaken. As has been stated, both sets of structures are 

 evaginations of surface epithelium, both move in toward the 

 median line at certain stages in their development and both 

 exhibit some degree of epithelial degeneration. But beyond 

 these superficial resemblances there is nothing to indicate the 

 existence of any genetic relation between them. At no period in 

 their development are they in continuity nor do they even re- 

 semble each other in histological appearance, unless it be at the 

 end of the seventh day, when the filaments have been crowded 

 in upon the tubercles just prior to their disappearance. The 

 differences which they exhibit may be summarized in the follow- 

 ing paragraph. 



The epithelial evaginations which constitute the ridges de- 

 velop from a broad zone of thickened ectoderm situated on either 

 side of the pectoral wall. The margins of each zone grade imper- 

 ceptibly into the adjacent ectoderm so that its limits are never 



