3IO LEIGH HOADI.KV. 



these is absent, these fibers have continued to grow into the 

 mesenchyme, forming, in places, nerve-like bundles. Lewis 

 (1907 d) describes a similar outgrowth of nerves from the brain 

 tissue of Rana sylvatica from which he had removed the optic 

 primordium. He states (page 464) : "These nerves extend from 

 a region of the brain which, under normal conditions, never gives 

 rise to peripheral nerves, or at least to nerves that leave the brain 

 in the region to run into the mesenchyme. . . . The injury in 

 the brain in these experiments was done long before the nerves 

 normally appear." The data presented here seem to indicate 

 that the regulation of the growth of the elements is dependent 

 to some degree on the structural environment, and that the dis- 

 organization produced by the injury during operation is sufficient 

 to modify the gross structure obtained in development. 



IV. DISCUSSION 7 



The independent power of growth and differentiation of other 

 embryonic parts has been studied to a limited extent by this 

 same method. Danchakoff (1921) has investigated the growth 

 and the differentiation of the blastoderm of the "primitive streak 

 stage" of the chick embryo after transplantation to the "allan- 

 toic" membrane. Blastoderms of "ten and more somites" from 

 which the posterior portion to the eighth somite had been 

 removed, were also grafted. In the blastoderms transferred to 

 the allantois at an early primitive streak stage, no differentiation 

 of organs appeared; the implanted parts remain as "islands of 

 undifferentiated tissue." Growths from blastoderms of a late 

 primitive streak stage showing a slight indication of the head 

 process, show groups of cells which "grow well and reach a high 

 degree of differentiation." Eye, nervous tissue, and notochord 

 are present, and in the cartilage which forms around the cord, 

 different parts of normal vertebrae are recognizable. Dancha- 

 koff concludes from her results, "the earlier a whole blastoderm 

 is transferred to the allantois, the less growth and differentiation 

 is obtained." While the principles involved in the growth ;uul 

 development of such transplantations are probably the same as 

 those applying to the independent differentiation of the isolatnl 

 organ primordium, the situation in grafts of such large portions 



