2 28 BIOLOGICAL LECTURES. 



cumstances determine the single facts of heredity. The change 

 in our attitude toward this problem is similar to that which has 

 taken place in psychology. Psychologists no longer try to give 

 a theory of the soul or of consciousness, but try to analyze the 

 various groups of psychical phenomena more or less independ- 

 ently of each other. 



It may be desirable to illustrate what we mean by the analyt- 

 ical study of heredity. From the &gg of the sea urchin only 

 one embryo originates. If we isolate the first two cleavage 

 cells, we get two embryos. This agrees very well with Jaeger's 

 chemical theory, but it does not agree with Weismann's. I 

 have made an experiment which seems to show that neither 

 Jaeger's nor Weismann's theory is correct. If the contents 

 of the ^%g be transformed into a double sphere before segmen- 

 tation sets in, either a dumb-bell-shaped blastula or two blastulae 

 are formed which are at first grown together, but as a rule are 

 separated later. // seems that a blastula originates by the 

 cleavage cells being forced to migrate to tJie limit between sea 

 water and egg or to the surface of the egg. Probably tropisms of 

 the cleavage cells, chemotropism, or stereotropism are involved 

 in this. It is easy to observe that the cleavage cells of the 

 sea urchin's egg can execute amoeboid motions. Hence we 

 have to deal with a circumstance which neither the theory of 

 Jaeger nor of Sachs nor of Weismann nor of any other author 

 could anticipate ; namely, the necessity of the cleavage cells 

 creeping to the periphery of the ^gg. 



We may select another circumstance, for instance, the 

 heredity of the arrangement of organs in an organism. From 

 the germ of Hydroids an organism develops which has stolons 

 at one end of its body and polyps at the other. This definite 

 arrangement is in many cases due to a fact which none of the 

 general theories of heredity could have predicted. We find in 

 many Hydroids that an excised piece of the stem forms stolons 

 if the end comes in contact with solid bodies, while the same 

 end forms polyps if it is surrounded by sea water. When 

 a germ or a larva settles down at the close of the swimming 

 stage, that end of the germ which is in contact with a solid 

 body will produce stolons, while the other end, that is in con- 



