BIPENNARIA LARVJE OF ASTERINA (PATIRIA) MINIATA. 123 



1. The first interpretation is well phrased by MacBride 1 as 

 follows : 



" The appearance of a right and left madreporic pore is the first 

 Indication of zvhat is really the key to the understanding of Echi- 

 noderm development, vis., the fact that the tivo sides of the larva: 

 originally gave rise to precisely similar organs, but that sonic of 

 these organs grew and developed on the left side while they 

 atrophied on the right, and that thus an asymmetry was 

 produced." 



This seems to be the natural interpretation of the facts, but 

 there are certain other facts that this interpretation does not 

 cover. In a larva of the sand-dollar Mcllita pcntapora, Grave 

 (1919) found that not only were the mesodermal structures, in- 

 cluding hydrocoel, pore-canals, etc., bilaterally repeated, but also 

 paired ectodermal pouches occurred which had no reference to the 

 water-vascular system, but were the primordia of the nervous 

 system. Grave is inclined to doubt the validity of interpreting 

 this extra ectodermal pouch as a reversion to an ancestral condi- 

 tion, and I would fully agree with him. The occurence of adult 

 starfishes with paired madreporic plates and stone canals also 

 weakens the phylogentic interpretation of double pores in larvae ; 

 for they are evidently strictly homologous structures. If the 

 double-plate condition in the adult is to be interpreted as a re- 

 version, of what is it an ancestral reminscence ? Surely not of an 

 ancestral starfish with radial symmetry of other organs, but bi- 

 lateral madreporic structures ! 



2. The second interpretation of paired madreporic structures 

 involves the idea that they are homceotic variations, in Bateson's 

 sense. Such variations may be viewed according to Grave " as 

 cases of perfected symmetry, the result of a long continued strain 

 due to imperfect balance, either morphological or physiological 

 or both, between the organism and its environment." I confess 

 that I am unable to see the force of this interpretation. It is 

 highly mystical in tone and savors of some internal perfecting 

 principle or " entelechy." If such a principle were operative, the 

 real problem would be to account for the failure of echinoderm 



1 " Text-book of Embryology," Vol. I., p. 466. 



