Anatomical characteristics of neck analyzed through development of tongue muscle

脊椎动物的身体主要由头部和躯干组成，头部和躯干与颈部相结合。为探讨颈部的解剖学特征，对不同脊椎动物颈部尤其是舌肌的发育进行了比较。在无颌鱼、七鳃鳗中，下鳃肌原基在咽弓的后方和下方发育，下鳃肌是颌口中与舌肌同源的肌肉。通过比较头神经嵴的分布和原基，未分化的肌肉祖细胞的舌肌似乎发展在头神经嵴后面的拱门。这些现象也观察到在胚胎中，并证实了由鸡鹌鹑嵌合体，其中头神经嵴移植。这些结果表明，舌肌原基起源于体节，以未分化的散在细胞形式迁移至弓后和弓下，并在头神经嵴中分化。然而，在鲨鱼中，原基已经分化成肌细胞，并作为几个分段的细胞流迁移到弓后面。这一观察结果表明，肌肉的发育是继发性的改变，导致鲨鱼在其进化和鳍肌肉的发展也不总是显示出祖先的状态。

Vertebrate body is mainly composed of head and trunk, which are combined with neck. To investigate the anatomical characteristics of neck, development of neck region, especially tongue muscles, was compared between various vertebrates. In agnathan, lamprey, anlage of hypobranchial muscle, homologous muscle to tongue muscles in gnathostome, developed behind and under pharyngeal arches. By comparing cephalic neural crest distribution and the anlage, undifferentiated muscle progenitor cells of the tongue muscles appear to develop in the cephalic neural crest just behind the arches. These phenomena were also observed in amniote embryos and confirmed by making chicken-quail chimera in which cephalic neural crest was transplanted. These results indicate that tongue muscles primordium originates from somites and migrates behind and under the arches as undifferentiated scattered cells and differentiates in the cephalic neural crest. In shark, however, the anlage was already differentiated to muscle cells and migrate behind the arches as several segmented streams of cells. This observation indicates that muscle development was secondarily changed in the lineage leading to shark in its evolution and development of fin muscle in shark also does not always shows ancestral state.