THE MOSAIC STAGE OF DIFFERENTIATION 205 



tionsof limb,^ ear,^ metanephros^ and other rudiments^ thus treated 

 have shown successful histological self-differentiation. Interesting 

 examples of chemical self-differentiation are found in isolated 

 portions of the skeleton. The cartilages of the palato-quadrate and 

 of the femur normally undergo ossification, whereas the distal 

 portion of Meckel's cartilage does not. The future histological 

 structure is already determined by the sixth day of incubation, al- 

 though there is then no visible distinction. The difference between 

 these two types of cartilage is revealed by cultivation in vitro, where 

 rudiments of the palato-quadrate and of the femur show a marked 

 synthesis of phosphatase, while that of the distal portion of Meckel's 

 cartilage does not : phosphatase activity is correlated with ossifica- 

 tion.^ If cultured long enough, ossification of a normal type super- 

 venes in the rudiments. It is worth mentioning that even in the 

 abnormal conditions provided by tissue-culture, in which the organs 

 are without blood supply, the volume of a chick femur will increase 

 up to about thirty times. 



Even extra-embryonic regions, such as the presumptive blood- 

 islands, develop histologically differentiated blood when cultivated 

 in vitro, ^ 



In some cases, at least, the determination imposed upon regions 

 in the mosaic stage of development concerns even the duration of 

 progressive differentiation and growth. The mesonephros of the 

 chick embryo normally undergoes regression at about the tenth day 

 of incubation, and if its rudiment is grafted on to the chorio- 

 allantoic membrane of another tgg, it will first differentiate the 

 typical mesonephric tissue, and then proceed to regress at about 

 the same time as regression would normally have occurred if it had 

 been left in place in the embryo." The time of regression in these 

 cases is, of course, in no way determined by the age of the host-egg 

 on to the chorio-allantois of which it is grafted (fig. lOo). 



The specific growth-capacities of the rudiments may also be 

 determined. In the intact bird, the right ovary is rudimentary and 

 the left is well developed. Four-day rudiments of the ovaries 



^ Strangeways and Fell, 1926. " Fell, 1928. 



3 Rienhoff, 1922. * Hoadley, 1924. 



'" Fell and Robison, 1929, 1930. ^ Murray, 1932. 

 ' Danchakoff, 1924. 



