144 DEVELOPMENTAL CHANGES IN THE EARLY EMBRYO 



Auditory vesicle 

 Epimeric mesoderm 



Notochord 



(splanchnic 



Three-dimensional representation of the tail bud stage of the frog embryo, 

 Rana pipiens. (Redrawn and modified after Huettner.) 



The Mesoderm and Its Derivatives. 



Embryonic, loosely dispersed presumptive mesoderm is known as 

 mesenchyme. At the end of gastrulation most of the frog meso- 

 derm is in the form of sheets of such loose cells extending in all di- 

 rections from the lips of the blastopore. However, the most anterior 

 mesoderm, that found in the head and pharyngeal regions, is in the 

 form of mesenchyme. 



Origin of the Arches. Lateral to the pharynx are developed 

 vertical concentrations of mesoderm known as the arches. Subse- 

 quently most of these will contain blood vessels and nerves but at this 

 stage they are merely condensations of mesenchyme. The most ante- 

 rior arch is anterior to the first endodermal pouch and ectodermal 

 groove, and is known as the mandibular arch associated with the de- 

 velopment of the jaw muscles. This was first seen ventral to the 

 sense plate on either side of the stomodeal and hypophyseal cleft. 

 Posterior to this mandibular arch is a parallel hyoid arch, and between 

 these arches is developed the rudimentary hyomandibular groove 

 (ectodermal) and pouch (endodermal) which never break through to 

 form the cleft. Posterior to the hyoid arch is the first endodermal 

 branchial pouch followed by the first mesodermal branchial arch; 

 the second branchial pouch followed by the second branchial arch; 

 and so on as the more posterior derivatives develop. The second arch 

 at this stage may not as yet have its mesenchyme clearly marked 

 off from the succeeding arches. 



