St 



CHAPTER 6 



of the bodj arc so grossly abnormal. Other 

 embryological work shows that proper so- 

 mite formation depends on the presence of 

 presumptive notochorda! tissue. When nor- 

 mal, presumptive notochord tissue is present, 

 its surrounding normal mesoderm is induced 

 to form cartilage and vertebral segments. 

 It seems reasonable to attribute the failure 

 of cartilage and vertebrae formation in T T 

 individuals to the failure o( its presumptive 

 notochord (which has lost the ability to de- 

 velop into notochord) to induce the differen- 



tiation of mesoderm. This explanation is 

 subject to test in certain experiments em- 

 ploying tissue culture. Using tissues from 

 normal mice, presumptive notochorda! tis- 

 sue, which has mesodermal tissue from the 

 same or another individual wrapped around 

 it. is placed in the tissue culture medium. 

 Development of cartilage and vertebral seg- 

 ments occurs under these conditions. More- 

 oxer, the mesoderm from normal embryos 

 also develops into cartilage and vertebral 

 segments, when surrounding presumptive 



BRACHY T+ 



25% 



die 10% days 



BRACHY T+ 



50% 



25% 



birth 



days 



MONSTER TT 



BRACHY T+ 



NORMAL + + 

 (Size Reduced) 



figure 6-13. Brachyury in the house mouse. (Courtesy of L. C. 

 Dunn; reprinted by permission of McGraw-Hill Book Co., Inc., from 

 Study Guide and Workbook for Genetics by I. H. Herskowitz. Copy- 

 right, 1 960.) 



