1-16 University of California Publications in Zoology. [Vol. 6 



1. INTRODUCTION. 



This paper embodies the results of an attempt to discover the 

 laws governing the growth and development of the zooids of the 

 Salpa chain. The investigation was instigated largely by a 

 reading of Dr. Raymond Pearl's memoir, "Variation and Differ- 

 entiation in Ceratophyllum " (Pearl, '07), in which the author 

 has outlined certain laws which he has found operative in the 

 growth of this plant. A sentence from one of his opening para- 

 graphs will give his point of view: "Since the condition of the 

 adult organism is, with respect to every character, the result of 

 a process of gradual development and growth, it is clear that in 

 order to gain anything approaching a satisfactory analysis from 

 the biological standpoint, we can not take the adult structure ;is 

 something given, but must investigate the laws according to 

 which the morphogenetic processes concerned in its production 

 operate." 



The general problem undertaken here is similar to Dr. Pearl's 

 but the difference in the material used makes necessary a differ- 

 ence in the method of attack. 



Dr. Pearl's problem was "to determine and formulate so far 

 ;is possible, the laws according to which differentiation with 

 growth occurs in Ceratophyllum" on the basis of the number of 

 hurts in tin leaf whorls of tin plant. Ceratophyllum bears a 

 large number of whorls, some with a larger and some a smaller 

 number of leaves. The whorls with different numbers of leaves 

 distribute themselves in a characteristic way. Quoting his further 

 statement: "We have. then, by analysis of the gross frequency 

 distribution for the plant as a whole, to investigate the biological 

 laws which lead to the production in this particular organism 

 of the characteristic distribution observed. In biometrical term- 

 inology, our problem is one of intra-individual variability." 1 

 Moreover, Pearl dealt with the adult organism. The plants were 

 still growing, it is true, but leaf-number even in the tiny leaflets 

 at the tip of the branch was already established, so that the 

 results were, strictly speaking, for the adult organism rather than 

 for differentiation with growth, as stated. 



The italics are mine. — M. E. J. 



