456 



Special Vertebrate Organogenesis 



axolotl, Balinsky found that presumptive 

 liver tissue grafted into the stomach region 

 could differentiate into stomach tissue. Like- 

 wise, presumptive stomach differentiated into 

 liver cells and pancreatic acini when grafted 

 into the regions of the latter tissues. These 

 experiments suggest that the stomach, liver 

 and pancreas have only a labile type of 

 determination in neurula stage Triturus. In 

 the pancreas, there is an interesting differ- 

 ence between the potencies of the dorsal and 

 ventral primordia in that only the former is 

 capable of differentiating islands of Langer- 

 hans (Wolf-Heidegger, '36). 



The influence of mesodermal derivatives on 

 the form of the digestive organs has been 

 studied particularly by Kemp ('51). Em- 

 bryos of R. pipiens at tail-bud stages were 

 transected at different levels and reared in 

 Holtfreter's sokition. In other experiments, 

 dorsal and ventral embryonic regions were 

 excised. The following conclusions were 

 drawn: development of a normal pattern of 

 intestinal coiling in anurans is dependent 

 upon the establishment of the vitelline cir- 

 culation, upon the regvilation of hydrostatic 

 pressure within the digestive tract and 

 coelom, and upon the restricted space of the 

 coelomic cavity. 



In the chick, chorioallantoic grafts indi- 

 cate that liver potency tissue exists in both 

 prospective dorsal and prospective ventral 

 embryonic regions until sometime between 

 the pre-somite and third somite stages; after 

 this, the liver is segregated in so far as the 

 potentialities for its development are elimi- 

 nated from the dorsal region (Rudnick, '35). 

 Segregation of a similar natvire occvirs at 

 a somewhat later stage for the pancreas. 



Liver differentiation in the chick appears 

 to depend upon an inducing action from the 

 heart rudiment, since liver is rarely present 

 in grafts lacking heart tissue whereas the 

 converse occurs quite often (Willier and 

 Rawles, '31). The inducing action occurs 

 particularly in grafts containing prospective 

 liver material; the heart inductor action 

 seems unnecessary when the liver develops 

 from grafted material of some other prospec- 

 tive value, e.g., from prospective dorsal re- 

 gions (Rudnick, '35). In amphibians, liver 

 differentiation is not dependent upon heart 

 development since liver can differentiate in 

 the absence of the heart in explants from 

 the gastrula stage (Holtfreter, '25). 



Embryos survive complete liver extirpa- 

 tion for a longer period than do larvae or 

 adults of the same species. When the opera- 



tion is done on embryos in the gill-formation 

 stage, svirvival is about two weeks for anuran 

 amphibians (Yamada, '33) and about four 

 weeks for urodeles (Copenhaver, '43). Devel- 

 opmental defects following embryonic hepa- 

 tectomy indicate that liver development af- 

 fects the formation of other embryonic struc- 

 tures through (a) a morphological relation- 

 ship and (^) a functional relationship. Ex- 

 amples of the first type include modifications 

 in the ventral mesentery and alterations in 

 the course of venous drainage from the in- 

 testine. These changes can be seen best when 

 the growth of a hepatectomized embryo is 

 maintained either by parabiosis with a nor- 

 mal embryo (Yamada, '33) or by liver tissue 

 implanted to the tail (Copenhaver, '43). 

 Defects resulting from functional changes 

 following liver extirpation include anemia 

 and retarded growth of the spleen (Copen- 

 haver, '43). The heart may be abnormally 

 small, particularly in anemic animals. Other 

 defects arising either directly or indirectly 

 from functional changes following hepatec- 

 tomy are distention of the pronephric canals 

 (Holtfreter, '25), hypertrophy of the pro- 

 nephros (Yamada, '33), and edema (Holt- 

 freter, '25; Yamada, '33). 



Numerous studies have established the 

 fact that there is a rapid restoration of liver 

 tissue following partial hepatectomy in 

 mammals. Higgins and Anderson ('31) have 

 shown that the liver of rats can regain its 

 normal size within two to three weeks after 

 a 70 per cent hepatectomy. According to 

 Newman and Grossman ('51), nucleic acid 

 supplements in the diet accelerate the rate 

 of regeneration. Higgins and Anderson noted 

 that restoration of mammalian liver tissue 

 differs from regeneration of the tadpole's tail; 

 in the former, there is no blastema of re- 

 generating tissue at the level of the cut. 

 When a lobe of the liver is removed, there 

 is a proliferation of cells and formation of 

 new lobules throughout the remaining lobes 

 but the extirpated lobe itself is not restored. 

 Considering the fact that most organs have 

 a much greater regenerative capacity in 

 amphibians than in mammals, it is surprising 

 to find that an opposite result has been re- 

 ported for the liver. Experiments on Bombi- 

 nator and on R. esculenta by Holtfreter ('25) 

 have confirmed the lack of liver regeneration 

 reported by Banchi ('06) for Bufo. Studies 

 on the salamander, Triturus viridescens, by 

 Jordan and Beams ('30), showed compensa- 

 tory hypertrophy bvit no proliferative activity 

 after semihepatectomy. After excision of 



