28 UNIVERSALITY OF DEGENERATIVE EVOLUTION 



(b) Our second group comprises the cases of 

 cellular adaptation which may be produced in the 

 course of embryonic life and may result in the for- 

 mation of an embryo from half or the quarter of 

 an egg. 1 



Here again the formation of organs takes place 

 under new and abnormal conditions, but there is 

 no " transformation " of these organs into fresh 

 structures exhibiting other characters than those 

 of primitive and typical organs. 



pieces are joined by an oblique segment, the trabeculse in this seg- 

 ment follow the direction from whence comes the greatest strain, a 

 different direction to that which they would have taken had the 

 inserted segment been placed parallel with the two broken pieces. 



Without entering upon explanatory theories concerning these 

 facts, it is important to notice that the adaptations of plants exhibit 

 different characters to those of animals. 



With plants it is only when a young organ is born under new 

 conditions that it exhibits new characters. This is not necessarily 

 the case with animals. In an animal organism the separation of 

 the young tissues from the old is not so noticeable as with plants, 

 as the organs undergo a continuous renewal and can always adapt 

 themselves more or less to new conditions. 



1 When the two cells resulting from the first division of a fertilized 

 egg of Ampliioxus are artificially separated, each cell may develop 

 directly into a complete individual. The same happens even when 

 the first four cells are separated from each other artificially. 



When this is effected in the case of Echinus or of an Amphioxus 

 embryo of eight cells, each cell developes as if it had remained a 

 part of the whole ; but when the blastular stage is reached, that 

 stage slowly completes itself. 



In the first case the cell adapts itself to the new conditions, in 

 the second case it is the blastular which does so. (See Analytische 

 Theorie der organischen Enturickelung. . Hans Driesch, Leipzig, 

 1894.) 



