PRIMORDIUM DEVELOPMENT 163 



This 1 have called stage 2. At stage 3 the anlage has become some- 

 what longer and the ciha are clearly visible in it but they have not 

 attained their final length. Even so, as Johnson observed, the cilia 

 begin beating on their first appearance, at first slowly and without 

 coordination. Oral cilia then attain their definitive length, and 

 transverse stripes in the rift indicate the formation of membranelles 

 which now beat slowly but in metachronal rhythm. The membran- 

 elles are at first very close together and they will produce a 

 membranellar band longer than the primordium as the distance 

 between them later increases (Stevens, 1903; Schwartz, 1935). 



As now deployed, there runs immediately to the right of the 

 membranellar row a pigment stripe and to the right of this a clear 

 band (the border stripe of Schuberg), and further to the right 

 another pigment stripe, which three will form the border stripes 

 of the frontal field. In the meantime, in an extensive area to the 

 right of these, considerable stripe multiplication has usually 

 occurred with formation of many kinetics separated by very fine 

 granular stripes, as Moxon first noted. It is these fine stripes which 

 will form the new frontal field, as well as the lining of the buccal 

 pouch and of the gullet in part. At stage 5 the posterior end of the 

 anlage enlarges a bit and begins to make a sharp bend to the right 

 in the initiation of mouthparts formation. A spiraling ingrowth of 

 the end of the primordium forms the gullet (stage 6). In this 

 invagination the terminal membranelles are carried down inside 

 the cell, as well as the posterior ends of the fine striping, and 

 shortly a gullet lined with bright refringent oral cilia and pigmented 

 ectoplasm is produced. It seems Hkely that there are some further 

 additions at this time to produce the complete lining of the gullet, 

 but this is not known for sure. 



At stage 7 the ectoplasm adjacent to the membranellar band and 

 just forward of the spiral gullet begins, in coeriileiis, to depress 

 and form the oral pouch as the anlage starts shifting into its final 

 position. With further development the entire anlage moves into 

 its final position, carrying the new stripes with it as the enclosed 

 frontal field (stage 8). In this migration the primordium which at 

 mid-stage was roughly parallel to the lateral striping comes to 

 assume a position at right angles to it and this may involve cutting 

 and shifting of stripes, as Moxon remarked, as well as compen- 

 sating growth in length of the stripes below the new mouthparts 



