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，与颈部结合在一起。为了研究颈部的解剖学特征，比较了不同脊椎动物的颈区发育，尤其是舌肌的发育。无齿鱼、七鳃鳗、鳃下肌原肌、舌口肌的同源肌，发育于咽弓后和咽弓下。通过比较头神经峰的分布和舌肌的原始肌原细胞，舌肌未分化的肌祖细胞似乎是在弓后的头神经峰发育的。在羊膜胚胎中也观察到了这些现象，并通过制作鸡-鹌鹑嵌合体来证实这一点，其中移植了头神经脊。这些结果表明，舌肌原基起源于体节，以未分化的散在细胞的形式迁移到弓后和弓下，分化于头神经脊。然而，在鲨鱼中，原始细胞已经分化成肌肉细胞，并作为几个分段的细胞流迁移到拱门后面。这一观察表明，肌肉的发育在谱系中发生了继发性的变化，导致了鲨鱼的进化，鲨鱼的鳍肌的发育也并不总是表现出祖先的状态。

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.