DIFFERENTIAL SUSCEPTIBILITY IN POLYCHETES 39 



is little larger than the egg, but the elongating trunk region is 

 relatively larger than in Nereis, and the development of the 

 first three trunk segments is accompanied by little reduction 

 of the head, so that the three-segmented larva is considerably 

 larger in relation to the egg than in Nereis. Later, however, 

 as additional segments arise, the head region undergoes marked 

 reduction. 



The figures of the different stages of each species are all drawn 

 as nearly as possible on the same scale, but the figures of the 

 different species are drawn with different magnifications, those 

 of Chaetopterus being on a considerably larger scale than the 

 others, and those of Arenicola being on a somewhat larger scale 

 than those of Nereis. Consequently the figures show the size 

 relations for different stages of each species, but not those be- 

 tween the different species. 



The size relations between egg and larva of each species depend, 

 in part, on the size of the coelomic cavity which is larger in 

 Chaetopterus than in the other forms and may be enlarged still 

 further by altering the metabolic relations between different 

 regions (fig. 17, A, B,). The proportions of the larvae also are 

 evidently determined by the metabolic relations between the 

 different regions for they too are altered by altering these re- 

 lations. In Nereis, for example, differential inhibition from the 

 beginning of development, that is, a greater inhibition of 

 the head than of the posterior region, produces larval forms like 

 figures 29 to 37 which, as regards general proportions of anterior 

 and posterior regions, approach the Arenicola type (figs. 48, 49). 

 On the other hand, differential inhibition, beginning at the young 

 trochophore stage in Arenicola, for instance, greater inhibition 

 of the posterior region than of the head, gives larval forms (figs. 

 51 to 56) whose proportions resemble somewhat those of certain 

 stages of Nereis (fig. 26) . Evidently the proportions of the three- 

 segmented larva depend, to a considerable degree, on the metab- 

 olic relations between the head, the body segments and the 

 posterior growing region. The Chaetopterus larva is somewhat 

 different in form, but its proportions can be altered in the same 

 ways. 



