CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION 523 



a. Head Mesoderm Derived from the Anterior Region of the Trunk 



The mesoderm of the branchial area in lower vertebrates, such as the sharks 

 and, to some degree, the amphibia, represents a direct anterior extension of 

 the mesoderm of the trunk (figs. 217D, E; 230D; 252E). It is divisible into two 

 parts: (1) a ventro-lateral region, the hypomeric or lateral plate mesoderm, 

 and (2) a dorsal or somitic portion. The latter represents a continuation into 

 the head region of the epimeric (somitic) mesoderm of the trunk. That por- 

 tion of the mesoderm of the branchial area which may be regarded specifically 

 as part of the mesoderm of the head proper is the mesoderm associated with 

 the mandibular and hyoid visceral arches, together with the hyoid and man- 

 dibular somites located at the upper or dorsal ends of the hyoid and man- 

 dibular visceral arches (fig. 217D, E). 



In the higher vertebrates (reptiles, birds, and mammals), the mesoderm 

 of the branchial region appears early, not as a continuous epithelium, as in 

 the shark and amphibian embryo, but as a mass of mesenchyme which wan- 

 ders into the branchial area from the anterior portion of the developing trunk 

 region (figs. 217F; 233B; 234B). This mesenchyme assumes branchial region 

 characteristics, for it later condenses to form the mandibular, hyoid, and more 

 posteriorly located, visceral arches. Also, mesenchymal condensations appear 

 which correspond to the pre-otic head somites formed in the early shark 

 embryo. For example, in the chick, there is an abducent condensation, which 

 corresponds to the hyoid somite of the shark embryo, and a superior oblique 

 condensation corresponding probably to the mandibular somite of the shark 

 embryo (cf. fig. 217D, F). (See also Adelmann, '27, p. 42.) Both of these 

 condensations give origin to eye muscles (Chap. 16). Somewhat similar con- 

 densations of mesenchyme which form the rudiments of eye muscles occur in 

 other members of the higher vertebrate group. 



b. Head Mesoderm Derived from the Pre-chordal Plate 

 The term pre-chordal plate mesoderm signifies that portion of the head 

 mesoderm which derives from the pre-chordal plate area located at the an- 

 terior end of the foregut. The pre-chordal plate mesoderm is associated closely 

 with the foregut entoderm and anterior extremity of the notochord in the late 

 blastula and gastrula in the fishes and amphibia. However, in reptiles, birds, 

 and mammals, this association is established secondarily with the foregut ento- 

 derm by means of the notochordal canal and primitive-pit invaginations during 

 gastrulation. (See Chap. 9 and also Hill and Tribe, '24.) 



{Note: It is advisable to state that Adelmann, '32, relative to the 19-somite 

 embryo of the urodele Ambystoma punctatum, distinguishes between a pre- 

 chordal mesoderm, which forms the core of the mandibular visceral arch, and 

 the pre-chordal plate mesoderm, which earlier in development is associated 

 with the dorsal anterior portion of the foregut entoderm. See figure 252E.) 

 During the period when the major organ-forming areas are being tubulated, 



