106 The Mechanistic Conception of Life 



Fig. 36 



Fig. 37 



Fig. 38 



Figs. 36, 37, and 38. — From nature. The origin of the pattern on the yolk 

 sac of a fish embryo {Fundulus heteroclitus). Fig. 36 is a drawing of the blood- 

 vessels at the surface of the yolk sac at an early stage of development. The black 

 pigment cells show no definite orientation in regard to the blood-vessels. Fig, 

 37, the same egg a few days later. Here we notice that some pigment ceUs show 

 a tendency to creep on the blood-vessels. Fig. 38, the same egg still a few days 

 later. The black pigment cells have completely crept on the blood-vessels and 

 formed a sheath around them. The red chromatophores are omitted in this 

 drawing. 



This was the first observation pro\'ing that tropisms play a role in the arrange- 

 ment of the organs in the body. 



