CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION 525 



the pre-chordal plate mesoderm separates as a mass of mesenchyme from the 

 antero-dorsal aspect of the foregut, anterior to the cephalic terminus of the 

 notochord (fig. 232G, H). It migrates forward as two groups of mesenchyme 

 connected at first by an interconnecting bridge of mesenchyme. Eventually 

 these two mesenchymal masses become separated and each forms a dense 

 aggregation of mesodermal cells over the mandibular visceral arch and just 

 caudal to the eye (fig. 252E). In the shark embryo and in the chick it gives 

 origin to the pre-mandibular somites (condensations) which probably give 

 origin to the eye muscles innervated by the oculomotor or third cranial nerves. 

 In Ambystoma, Adelmann ('32, p. 52) describes the pre-chordal plate meso- 

 derm as giving origin to "the eye muscles" and "probably much of the head 

 mesenchyme ahead of the level of the first (gill) pouch, but its caudal limit 

 cannot be exactly determined." Thus it appears that a portion of the head 

 mesoderm in the region anterior to the notochordal termination is derived 

 from the pre-chordal plate mesoderm in all vertebrates. 



c. Head Mesoderm Contributed by Neural Crest Material 



A conspicuous phase of the development of the head region in vertebrate 

 embryos is the extensive migration of neural crest cells which arise in the mid- 

 dorsal area as the neural tube is formed (Chap. 10; fig. 222C-F). Aside 

 from contributing to the nervous system (Chap. 19), a portion of the neural 

 crest material migrates extensively lateroventrally and comes to lie within the 

 forming visceral (branchial) arches, contributing to the mesoderm in these 

 areas (figs. 222C-F; 230D, F). Also, consult Landacre ('21); Stone ('22, 

 '26, and '29); and Raven ('33a and b). On the other hand, Adelmann ('25) 

 in the rat and Newth ('51 ) in the lamprey, Lampetra planeri, were not able 

 to find evidence substantiating this view. However, pigment cells (melano- 

 phores) of the skin probably arise from neural crest cells in the head region 

 of all vertebrate groups. 



d. Head Mesoderm Originating from Post-otic Somites 



There is good evidence that the musculature of the tongue takes its origin 

 in the shark embryo and lower vertebrates from cells which arise from the 

 somites of the trunk area, immediately posterior to the otic (ear) vesicle, from 

 whence they migrate ventrad to the hypobranchial region and forward to 

 the area of the developing tongue (fig. 253). In the human embryo, Kingsbury 

 ('15) suggests this origin for the tongue and other hypobranchial musculature. 

 However, Lewis ('10) maintains that, in the human, the tongue musculature 

 arises from mesenchyme in situ. 



3. Origin of the Mesoderm of the Tail 



In the Amphibia, the tail mesoderm has been traced by means of the Vogt 

 staining method to tail mesoderm in the late blastular and early gastrular 



