498 C. M. Child 



where a part after isolation is incapable of becoming a whole, 

 while the remaining parts are capable of replacing it, there is 

 reason for believing that the correlations are more or less one- 

 sided, i. e., the part in question has been so greatly modified by 

 the past or present correlations arising from other parts that it has 

 lost its totipotence and can never become a whole, but the correla- 

 tions arising from this part have not been sufficient to modify the 

 other parts of the system to an equal extent. Examples under 

 this head are the appendages of arthropods, amphibia, etc. 



One other point discussed by Holmes requires brief considera- 

 tion: in his first paper he selected the regulatory development of a 

 head in Planaria as an illustration of the working of social pres- 

 sure. In his discussion of this case differentiation is regarded as 

 proceeding from the cut surface distally, in consequence of the 

 social pressure exerted on the new parts by the old (Holmes '04, 

 pp. 282, et seq.). In my criticism of this point (Child '06a, p. 

 421, et seq.), I called attention to the fact that in Planaria, and in 

 other forms as well, differentiation of the regenerating tissue actu- 

 ally proceeds in the opposite direction, i. e., from the tip toward 

 the base. In reply to my criticism Holmes ('07, pp. 427, 428) 

 points out that the first visible differentiation is not necessarily 

 the first actual differentiation, that ''before any external features 

 are produced in the development of a hmb the main outhnes of 

 its differentiation may have been established through influence 

 proceeding from its basal part, after which the tip might differen- 

 tiate more rapidly than the intervening portion and the other visi- 

 ble features of structure appear successively toward the base." 

 He also points out that in many cases the visible differentiation 

 is centrifugal rather than centripetal and cites the case recently 

 described by Zeleny ('07) of the antennule of Mancasellus, in 

 which visible differentiation at first proceeds from the base 

 toward the tip, but later in the opposite direction. He continues: 

 ^'But granting that, in many cases, differentiation actually begins 

 at the extremity and works toward the base of the regenerating 

 organ, the process is not inconsistent with the point of view here 

 set forth. We may suppose that the influence of the environ- 

 ment causes the extremity of an organ to begin to differentiate 



