Expansion of the area vasculosa of the chick after removal of the ectoderm

ABSTRACT The role of the ectoderm in the expansion of the mesoderm in the area vasculosa of the chicken embryo was studied. The basement membrane of the ectoderm was found to constitute a substratum for the expansion of both layers of mesoderm, since (a) the somatic mesoderm, particularly at its margin, adheres to the basement membrane, and (b) the somatic and splanchnic mesoderm adhere to each other throughout most of the area opaca. Following removal of the ectoderm from the outer surface of the basement membrane, movement of the underlying mesoderm along its inner surface stopped. Mean expansion of the mesoderm in these cases was zero. Following removal of both ectoderm and basement membrane, expansion of the underlying mesoderm was normal in amount. Experimental changes in the ectodermal substratum can thus stop movement of the associated mesoderm, but the role which the substratum normally plays in mesodermal expansion remains unclear.