Heart, Blood Vessels, Blood, and Entodermal Derivatives 



455 



to develop from, foreign ectoderm trans- 

 planted into the stomodeal region. Balinsky 

 concludes that there is a regional factor in 

 the presvimptive mouth ectoderm but that 

 determination occurs gradually. At the 

 neurula stage, the presumptive mouth ecto- 

 derm has only a labile determination and 

 it will not differentiate without an inductive 

 action from the underlying entoderm. 



Tooth development in some amphibians 

 differs from that in mammals in regard to 

 the origin of the enamel organs. Experiments 

 by Adams ('24, '31) showed that in A. 

 punctatum the enamel organs of some tooth 

 germs arise from ectoderm, others from 

 entoderm. On the other hand, Woerdeman 

 failed to find proof for the formation of 

 teeth from entoderm in xenoplastic grafts 

 between T. taeniatus and the axolotl 

 (Woerdeman and Raven, '46). 



Studies on Amblystoma by Adams ('24, 

 '31) supported the view that the pulp of the 

 tooth germ is mesectodermal (neural crest) 

 in origin. Her experiments also raised the 

 question whether the entoderm or mesecto- 

 derm initiates tooth formation. Among those 

 who have studied this question fiu'ther, 

 Woerdeman and Raven ('46) concluded that 

 there is a reciprocal indvictive influence — that 

 the papilla "induces the epithelium to form a 

 tooth-germ, whilst the size of the papilla de- 

 pends on the enamel-organ." Horstadius ('50) 

 demonstrated the reciprocal relationships 

 more specifically by experiments in which 

 tissues from the head were grafted in differ- 

 ent combinations into the trunk. He found a 

 failure of tooth development in the following 

 grafts: oral entoderm plus oral ectoderm, 

 oral entoderm plus neural crest, and oral 

 ectoderm plus neural crest. Teeth developed 

 in grafts of oral entoderm plus neural crest 

 plus oral ectoderm. Studies on the man- 

 dibular arch by Levy and Detwiler ('51, and 

 personal communication) are essentially in 

 agreement with the views of Horstadius. 

 Likewise, in vitro studies by Wilde ('51) 

 show that it is necessary to explant cranial 

 neuroepithelium along with stomodeal ecto- 

 derm and foregut entoderm for teeth to 

 develop. 



Role of Entoderm in Gill Formation. Har- 

 rison ('21) studied the respective roles of 

 branchial ectoderm and mesoderm in gill 

 formation by extirpation and transplantation 

 experiments. He established the following 

 facts: (1) the branchial ectoderm of A. 

 punctatum is specifically gill forming but 

 an ectodermal region surrounding the gills 

 also has the potency to form gills in a 



diminishing intensity as the distance from 

 the gills increases; (2) the specific gill pat- 

 tern is not laid down in the ectoderm; (3) 

 the specific pattern must be determined by 

 the deeper layers. In the absence of experi- 

 ments on the entoderm, the respective roles 

 of mesoderm and entoderm were not clear 

 but it was suggested that the entoderm did 

 not play merely a passive role. Heteroplastic 

 gill transplantations made by Harrison ('29) 

 showed that gills resembling those of the 

 donor species occurred only when the whole 

 gill complex was transplanted, entoderm 

 included. The importance of the branchial 

 entoderm was demonstrated in experiments 

 on tail-bud stage Amblystoma embryos by 

 Severinghaus ('30) in which it was found 

 that a gill fails to develop after removal of 

 its entoderm. 



It is well established that neural crest cells 

 contribute to the formation of the visceral 

 arch skeleton (Landacre, '21; Stone, '22; and 

 others). It has also been shown that the 

 entoderm of the oral region acts as an in- 

 ductor on the migrating neural crest cells 

 for the formation of cartilage. Following ex- 

 perimentally produced defects in the oral 

 region, Balinsky ('47b) found a correlation 

 between the aiTangement, number and size 

 of the visceral cartilages and the presence of 

 entoderm of the pharyngeal pouches. In 

 transplantations of head tissues to the trunk, 

 Horstadius ('50) found that neural crest 

 transplanted with oral entoderm will pro- 

 duce cartilage whereas neural crest grafted 

 with oral ectoderm alone will not. 



Mid-gut Derivatives— Stomach, Intestine, 

 Liver and Pancreas. In amphibians, presump- 

 tive entodermal organ rudiments appear to 

 be determined relatively early. The hetero- 

 topic transplantation experiments on Bombi- 

 nator by Holtfreter ('25) showed that the 

 liver and pancreatic primordia are capable 

 of self-differentiation in the late gastrula 

 stage. He found that the liver primordium 

 itself is not a mosaic — it is not regionally 

 determined — and a gallbladder can develop 

 in grafts from either cephalic or caudal por- 

 tions of the liver rudiment of tail-bud em- 

 bryos. Whether one presumptive organ 

 rudiment (e.g., stomach) can regulate to 

 form another (eg., liver) is controversial. 

 According to one view (Holtfreter, '25, '38; 

 Kemp, '46, '51; and others) considerable 

 regulation may occur in gross morphology 

 but not in histological structure. According 

 to another view (Kusche, '29; Balinsky, '38, 

 '47b), histological regulation can occur. In 

 xenoplastic grafts between Triturus and the 



