SECOND LECTURE. 



CLEAVAGE AND DIFFERENTIATION. 



E. G. CONKLIN. 



Philosophically, the most important problems of biology are 

 those which concern the origin of a new individual, the genesis 

 of a living organism. To the great problem of development 

 has been devoted the earnest thought of philosophers and 

 scientists of every age. The mystery which hangs about the 

 process of progressive and coordinated differentiation by which 

 the egg cell is transformed into the adult never loses its 

 charm nor ceases to be a mystery. 



Recent years have witnessed the most remarkable activity in 

 this field, and the views now extant are so numerous, so difficult 

 of concise representation, and have been so frequently discussed 

 that it seems undesirable to dwell upon many of them here. 

 In this lecture I shall present some observations and conclu- 

 sions derived from a study of the normal development of certain 

 gasteropods and shall attempt to apply these results to some 

 of the current theories of development. Unfortunately, the 

 nature of this material is such as to render direct experiment 

 difficult and in most cases unsatisfactory. Observation, how- 

 ever, is still a valuable method in biology, and it has by no 

 means revealed all that it can, either as to the course or the 

 causes of development. It seems to be assumed in certain 

 quarters that we already know all the important phenomena of 

 normal development and that mere observation is, therefore, a 

 useless and antiquated method. If the time ever comes when 

 every step in the normal development of a single individual 

 is known, the causes of development will not be far to seek. 

 There is no such sharp distinction between observation and 



