Organisms from Eggs 147 



We may finally allude briefly to the fact that when 

 once a number of tissues are differentiated each one 

 may influence the other by calling forth tropistic re- 

 actions. Thus the writer showed that in the yolk sac 

 of the fish Fundulus the pigment cells lie at first without 

 any definite order but that they gradually are compelled 

 to creep entirely on the blood-vessels and form a sheath 

 around them with the result that the yolk sac assumes 

 a tiger-like marking. 1 Driesch 2 has pointed out that 

 the mesenchyme cells are directed in their migration; 

 and it seems that the direction of the growth of the 

 axis cylinder is determined by the tissues into which 

 it grows. The idea of tropistic reactions in the forma- 

 tion of organs has been discussed by Herbst. 3 



6. As a consequence of further changes definite 

 anlagen or buds originate later in the embryo which 

 are destined to give rise to definite organs. Thus in 

 the tadpole early mesenchyme cells are formed which 

 are the anlagen for the four legs, which will grow out 

 under the proper conditions. These anlagen are 

 specific inasmuch as from the anlage of a foreleg only 

 a foreleg, and from the anlage for a hindleg only a hind- 

 leg, will develop. Braus 4 has proved this by trans- 



1 Loeb, J., Jour. Morphol., 1893, xiii., 161; The Mechanistic Con- 

 ception of Life. Chicago, 1912, p. 106. 



a Driesch, H., Science and Philosophy of the Organism, L, p. 104. 



3 Herbst, C., Formative Reize in der tierischen Ontogenese. Leipzig, 

 1901. 



* Braus, H., Munchener Med. Wochnschr., 1903, I (II.), No. 47, p. 

 2076. 



