38 C. M. CHILD 



resents the final developmental result of the existence of the 

 primary gradient. 



If this interpretation be correct, the segmental region of the 

 annelid body passes through three successive physiological stages : 

 in its origin and earliest development it represents the region 

 of lowest metabolic activity in the egg; second, it becomes the 

 most active region of the larva and gives rise to segments in which 

 a developmental gradient decreasing in rate anteriorly appears'; 

 third, development of the nervous system brings about a phys- 

 iological integration of these segments, and a gradient of nervous 

 stimulation decreasing in rate posteriorly from the first segment 

 appears. This final gradient may, sooner or later, extend to 

 the extreme posterior end of the body, or the developmental 

 gradient may persist in a longer or shorter posterior region, as in 

 the six-segmented Arenicola and inmost oligochetes (Hyman, '16). 



Certain differences between the different species 



In the killing experiments the egg, the blastomeres, and the 

 cells of later stages of Chaetopterus undergo marked swelling 

 and separate from each other very completely before death, in 

 the agents used, while in Nereis and Arenicola almost no swelling 

 and cell separation occurs, and death takes place with little 

 change in appearance. This difference in behavior is evidentl 

 due to some differences in protoplasmic constitution, but the 

 experiments throw no light on the question of its nature. 



Another interesting difference appears in the size relations 

 between egg and larva. In Chaetopterus the egg is much smaller 

 than the eggs of Nereis and Arenicola, but the young trochophore 

 is distinctly larger, the fully developed elongated larva is much 

 larger than the egg, and the decrease in size of the head region 

 is slight. In Nereis, where the egg is large, the young trochophore 

 differs but little in size from the egg, and after posterior elongation 

 begins, the head region undergoes a very marked reduction and 

 the three-segmented Nereis larva is but little larger than the egg. 

 Arenicola is intermediate between Nereis and Chaetopterus in 

 these respects. The egg is larger than that of Chaetopterus 

 and smaller than that of Nereis, and the young trochophore 



