324 MEAD. 



EXPLANATION OF PLATE XVIII. 



Scolecolepis viridis Verrill. 



Fig. 105. From life, side view : first polar globule,/.^. ; m, egg membrane. 



Fig. 106. From left: 2-cell stage, vegetative pole. Precocious formation of 

 cross-furrow. 



Fig. 107. From life : three cells, seen from animal pole. 



Fig. 108. From life : 4-cell stage, seen from animal pole. 



Fig. 109. From life : animal pole and side. 



Fig. iio. From life : i6-cell stage. 



Fig. III. Vegetative pole : same stage. 



Fig. 112. i6-cell stage: cells tinted brown correspond to the primary 

 trochoblasts of Amphitrite and Clymenella. 



Fig. 113. Lower hemisphere, after division of cells seen in Fig. no: from 

 life. 



Fig. 114. From animal pole : cells tinted brown as before ; cells a^-^, b^-^, and 

 <?■•'■ correspond to the secondary trochoblasts of Amphitrite and Clymenella. 



Fig. 115. Trochophore with three trunk segments; eye, eye-spot; prot, 

 prototroch ; par., paratroch ; m., mouth ; /, //, ///, trunk segments. 



Fig. 116. Optical section, side: muc, mucous gland; prot., prototrochal 

 cells ; ent., entoderm ; mes., mesoderm. 



