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，躯干与颈部相结合。为了探讨颈部的解剖学特征，本文比较了不同脊椎动物颈部尤其是舌肌的发育情况。在七鳃鳗中，鳃下肌肉的分支，与舌肌同源的肌肉，发育在咽弓的后面和下面。通过比较头神经嵴的分布和标本，舌肌未分化的肌祖细胞在舌弓后的头神经嵴发育。这些现象也在羊膜胚胎中观察到，并通过移植头神经嵴的鸡-鹌鹑嵌合体得到证实。这些结果表明舌肌原基起源于体小体，作为未分化的分散细胞迁移到弓后和弓下，并在头神经嵴中分化。然而，在鲨鱼中，胶原蛋白已经分化为肌肉细胞，并迁移到弓的后面，形成几条细胞流。这一观察结果表明，在鲨鱼进化过程中，肌肉的发育是继发性的，鲨鱼鳍肌肉的发育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.