Heart, Blood Vessels, Blood, and Entodermal Derivatives 



457 



more than one-half of the liver, the lobular 

 remnants were apparently incapable of ef- 

 fective restoration. 



Onset of Liver Function. Functional dif- 

 ferentiation of liver tissue has been studied 

 more completely for the chick than for other 

 species. Bile secretion appears on about the 

 sixth day of incubation (Sandstrom, '34). 

 On about the seventh day, appreciable 

 amounts of glycogen are found in the liver 

 although the pancreatic islands do not ap- 

 pear before the eleventh day (Dalton, '37b). 

 From the developmental seqvience just cited 

 and from the fact that glycogen is stored by 

 embryonic liver cells cultivated in vitro 

 (Nordmann, '29), it is obvious that the syn- 

 thesis and storage of glycogen in the embry- 

 onic liver is not dependent upon insulin. 

 However, one notes a more adult type of 

 glycogen distribution in the liver after the 

 islands of Langerhans appear. Data on the 

 excretion of uric acid indicate that the liver 

 begins to function in protein metabolism on 

 about the seventh day (Fiske and Boyden, 

 '26). Cholesterol is normally not evident 

 in liver tissue vmtil the eleventh day of 

 incubation, but an earlier capacity for lipid 

 metabolism is indicated by the fact that 

 cholesterol appears on the seventh day in 

 embryonic hepatic cells grafted to the chorio- 

 allantois of older embryos (Dalton, '37a). 

 Thus the available evidence indicates that 

 most of the multiple functions of the liver 

 cell appear at about the same time. 



Although the hepatic cell is capable of 

 function at an early age, it does not assume 

 its full degree of function until relatively 

 late. In the chick, glycogen storage in the 

 extraembryonic tissues exceeds that in the 

 liver until the seventeenth day of incubation, 

 i.e., until 82 per cent of the incubation 

 period has elapsed (Needham, '31). In 

 mammals, the glycogenic function occurs 

 chiefly in the placenta until relatively late 

 in development. For example, the glycogen 

 content of the rat liver first exceeds that of 

 the placenta after the elapse of 75 per cent 

 of the total gestation period (Corey, '35). 



Asymmetry and Situs Inversus. The present 

 discussion will be confined to asymmetry of 

 the heart and viscera; for an analysis of 

 other aspects of symmetry in the developing 

 embryo, reference may be made to Harrison 

 ('45). 



Questions related to the development of 

 normal visceral asymmetry and to situs in- 

 versus have been studied extensively by the 

 experimental method, particularly in am- 



phibians. Spemann, ('06) showed that a 180 

 degree rotation of the middle part of the 

 medullary plate and its underlying meso- 

 derm and entoderm frequently produced 

 situs inversvis of the gut and heart. Nvimer- 

 ous investigators have confirmed and ex- 

 tended this finding. Experiments by von 

 Woellwarth ('50) are particvilarly significant. 

 In a control series of animals with a part 

 of the medullary plate and its underlying 

 tissue reimplanted with normal orientation, 

 he found visceral situs inversvis in about 

 the same frequency as in another series of 

 animals with a similar block of tissue rotated 

 through 180 degrees. Working on the hypoth- 

 esis that the unexpected results in the non- 

 rotated reimplantations were caused by tis- 

 sue damage, von Woellwarth made unilateral 

 defects in the different germ layers in gas- 

 trula and neurula stage embryos. The follow- 

 ing results were obtained from neurula stage 

 operations: defects in the left side of the 

 mesoderm gave weak inversion effects (9 

 per cent), defects of the right mesoderm 

 gave no inversion effects, defects in ento- 

 derm gave strong inversion effects (about 

 equal on the two sides — 22 and 21 per cent, 

 respectively, and defects in mesoderm and 

 entoderm gave a strong effect for the left 

 side (41 per cent) and a medium effect for 

 the right (20 per cent). In gastrula stage 

 operations, defects in the presumptive meso- 

 derm of the left side gave strong inversion 

 effects (50 per cent), defects of right meso- 

 derm gave weak effects (8 per cent) and 

 defects of presumptive entoderm of both 

 sides gave weak effects (6 per cent). 



The experiments of von Woellwarth show 

 that situs inversus occurs with the greatest 

 frequency after defects to the mesoderm of 

 the left side at the gastrula stage. He believes 

 that the results indicate an asymmetry of 

 the organizer. In this connection, he notes 

 that Goerttler ('28) argvied for a physio- 

 logical asymmetry of the mesoderm. The 

 latter reported that explants of presumptive 

 heart mesoderm from the left side of medul- 

 lary plate stage urodeles developed pulsa- 

 tions within a few days whereas explants 

 from the right side did not. On the other 

 hand, Holtfreter ('33) found that pulsations 

 began at about the same time in right and 

 left heart anlage explanted from neurula 

 stage axolotls. Nevertheless, there is support 

 for the idea of an asymmetry in the two 

 sides of the heart anlage. Bacon ('45) found 

 that both sides of the bilateral heart rudi- 

 ment of medullary plate stage Amblystoma 



