The Limit Between Ectoderm and Entoderm. 65 
The presumption, from the standpoint of nerve distribution, is all in 
favor of the origin of taste buds from entoderm. 
Summary oF Resvutts. 
1. In Amblystoma punctatum the relations of preoral entoderm, 
premandibular somites, hypophysis and neural tube are essentially 
the same as in selachians. There are strong indications that the 
preoral entoderm and hypophysis together constitute the vestige of a 
palzeostoma. 
2. The entoderm surrounding the anterior part of the archenteron 
is compressed and the mouth remains closed until a relatively late 
period. The location of the palzeostoma and of the neostoma can be 
traced continuously to the time when the mouth opening is formed. 
3. The formation of the mouth opening includes three phases: 
(1) the disappearance of the ectoderm over the mouth area; (2) the 
relatively long period of solid entoderm; (3) the progressive forma- 
tion of the oral cavity from behind forward as a cleft in the ento- 
derm. 
4. The deep layer of ectoderm gathers.into a thickening or collar 
around the mouth plate and this grows in around the solid entoderm, 
thus forming the dental ridges. From these are formed the maxillary, 
mandibular and vomerine teeth. The ectoderm cells and teeth are 
separated from the mouth cavity by a layer of entoderm. When the 
teeth are fully formed they pierce this entoderm to enter the mouth. 
5. The taste buds are all formed in entoderm. The entoderm lines 
the whole mouth cavity to the very lips until some days after the 
taste buds are well formed. The taste buds in all parts of the mouth 
and pharynx are formed by re-arrangement of yolk-laden entoderm 
cells. There is no evidence that any taste buds in Amblystoma 
punctatum are of ectodermal origin. The relations of entoderm and 
ectoderm are such that it is impossible that any of them should 
arise from ectoderm in this animal. 
