I20 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



A few data on differential susceptibility of developmental stages are 

 available (Child, igijd; Hyman, 1916a). Susceptibility of the egg of 

 Chaetopterus pergamentaceus before and after fertilization decreases from 

 apical to basal pole (Fig. 37, A). During maturation susceptibility of 

 the apical region is relatively high. The basipctal gradient persists dur- 

 ing cleavage and after gastrulation in the ectoderm, but in early motile 

 stages susceptibility increases in the dorsiposterior region, slightly anterior 

 to the extreme posterior end, and in the early trochophore this region be- 

 comes more susceptible than the apical region and extends ventrally 

 around the larva (Fig. 37, 5, C). This is the region of the so-called "so- 

 matic plate," which gives rise to the trunk segments. The region of the 



A 



B 



Fig. 37, A~C. — Differential susceptibility of Chaetopieriis pergamentaceus egg and larvae. 

 A, basipetal gradient in undivided egg; B, C, stages of cytolysis in trochophore (from Child, 

 1917J). 



developing mouth also shows increased susceptibility at this stage (Fig. 

 37, C). Nereis limbata and Arenicola cristata, also polychetes, show a simi- 

 lar simple gradient from apical pole in early stages; later, as the somatic 

 plate becomes active, its susceptibility increases. The cells of the Nereis 

 prototroch also show increased susceptibility as their jciiiary activity de- 

 velops. What may be called a U-shaped gradient is apparently character- 

 istic of annelids from, or somewhat before, the stage when development of 

 trunk segments begins, throughout life, or as long as segment formation 

 continues. In these polychetes it originates in the trochophore. The term 

 "U-shaped" is merely a convenient designation for a gradient pattern in 

 the longitudinal axis with two high ends and a low region between them; 

 strictly speaking, such a gradient is not U-shaped but consists of two arms 

 with differential in opposite directions. It is present at least temporarily in 



