464 



DEVELOPMENT OF PRIMITIVE BODY FORM 



NEURAL CREST CELLS 



VAGUS LATERAL LINE 

 PLACODES 

 EPI BRANCHIAL 

 PLACODES 

 OF N VAGUS 

 ^OTIC VESICLE 



-!V LATERAL 

 LINE 

 PLACODE 



LONGITUDINAL 



ECTODERMAL 



THICKENING 



ECTODERMAL 

 THICKENING 

 HYOMANOIBULAR 

 CLEFT 



EPIBRANCHIAL PLACODE 



GLOSSOPHARYNGEUS 



FACIALIS 



D 



SUPRAORBITAL 



GROUP OF SENSE 



ORGANS 



MIOBODY 



LINE OF SENSE 

 ORGANS 



Fig. 222. Neural crest cells in Ainby stoma piinctatum. (A and B from Johnston: 

 Nervous System of Vertebrates, Philadelphia, Blakiston, '06; C-F from Stone: J. Exper. 

 Zool., '35.) (A) Transverse section of early neural tube of Ambystoma, neural crest 

 cells located dorsally and darkly shaded. (B) Later stage than (A), showing relation 

 of neural crest cells, epidermis, and neural tube. (C-F) Neural crest cells stippled, 

 placodes of special lateral line sense organs and cranial nerve ganglia shown in black. 

 The neural crest cells arise from dorsal portion of neural tube at points of fusion of 

 neural folds and migrate extensively. A considerable portion of neural crest cells descends 

 upon the mesoderm of visceral arches as indicated in (D-F) and contributes mesodermal 

 cells to these arches, where they later form cartilaginous tissue. 



of a delicate system of vessels which function for the transport of the circula- 

 tory fluid or blood. The formation of these blood vessels begins below the 

 forming entodermal tube as two, subenteric (subintestinal) tubes or capillaries. 

 These capillaries grow forward below the anterior portion of the forming 

 digestive tube. Near the anterior end of the latter, they separate and pass 

 upward on either side around the gut tube to the dorsal area, where they come 

 together again below the notochord and join to form the rudiments of the 

 dorsal aortae. The latter are two delicate supraenteric capillaries which ex- 

 tend from the forming head area caudally toward the trunk region. In the 



