The rearrangement of the cranial part of embryonal body in chick studied by linear marking. II. Relations of the neck, thorax and shoulder.

Abstract

The linear marking method was employed for studying morphogenetic movements of tissues involved in formation of the cranial part of the chick embryonic body. A total of 120 chick embryos marked on days 3-5 were used in this study with the following results: The cranial shift of axial structures and somites with respect to the other components of the cranial part of the embryonic body, described in our earlier papers, has been proven. This shift is considered crucial from the point of understanding and proper interpretation of the following formative processes: The prospective mesenchyme of the wall of the thorax shifts continuously in the cranial direction. The rate of the shift of the mesenchyme in the dorsal part of the developing body wall appears considerably lower than that of the somites. The cranial shift of the mesenchyme of the ventral part of the body wall is directed ventromedially and proceeds more rapidly than in the dorsal part of the body wall. The ventromedial deviation of the cranial shift is gradually increased in the craniocaudal sequence. In this way, the mesenchyme of the lateral body wall replaces step by step the membrana reunions to cover the heart, rapidly growing lungs and the liver. The presumptive shoulder shifts cranially at the rate similar to that of the somites. In this way, the shoulder slides over the body wall mesenchyme formerly located in the more cranial position. The shoulder undergoes rotation by the mechanism which is discussed.