FINE STRUCTURE 2g 



(a) Frontal field 



The granular or pigmented stripes of this area are narrow and 

 the ciHary rows correspondingly close together, not only because 

 they are ventral fine stripes shifted forward, but also because there 

 is usually an extensive formation of new stripes within the arc of 

 the developing membranellar band. Some of the resulting pigment 

 stripes in coeruleus may be so narrow as to consist of only a single 

 row of granules. The disposition of the frontal striping follows 

 roughly the curve of the membranellar band (Fig. i). Some of the 

 stripes end at the border, while those nearest the oral side continue 

 as the lining of the oral pouch and proceed downward and spiraling 

 into the depths of the gullet. 



Like their lateral progenitors, clear stripes of the frontal field 

 bear rows of small cilia, and contractile myonemes which are said 

 to be finer than those of the lateral body wall and without 

 branchings (Dierks, 1926a). Stevens (1903) thought that the frontal 

 stripes can multiply in situ and not only alongside the developing 

 primordium. This now seems unlikely because stripe splitting 

 within the field is not observed. Likewise, when for some reason 

 the stripes fail to increase adjacent to the anlage, the frontal field 

 is then deficient and remains so until corrected by a later re- 

 organization or re-regeneration. 



The frontal stripe area is bordered by a wider clear stripe 

 followed by a bordering pigment stripe and finally the membranel- 

 lar band itself (see Fig. i). According to Maier (1903), who studied 

 niger but says that coeruleus is the same, there is a marginal ciliary 

 row which neither Schuberg nor Johnson noticed on the clear 

 border stripe, but no myoneme underlies it. Schroder (1907) 

 seems to have been the first to mention the pigmented border 

 stripe in coeruleus though Schuberg (1890) showed it in his 

 drawings ; I have found it along the right side of the developing 

 oral primordium when presumably the clear border stripe is also 

 laid down. 



(b) Oral pouch 



In some species (e.g., introversus and roeseli) the frontal field 

 simply dips downwards like a ramp and forms a curved trough 

 leading to the opening of the gullet. In coeruleus there is a definite 

 in-pocketing such that the pouch curves back under the frontal 



