EMBRYOLOGY OF THALASSEMA MELLITA 185 



smaller than the macromeres and, as usual, are all entomeres. 

 The entoblast-plate, therefore, consists of 1 1 cells, namely, the 

 four macromeres, the fifth quartet, and the anterior and lateral 

 members of the fourth quartet (Text-Fig. 4, A). Gastrulation 

 begins when the embryo contains about 150 cells. 



3. Special Considerations. 



a. TJie Prototroch. — The primary prototroch in Tlialassevia 

 agrees in origin, cell by cell, with that oi Anipliitrite, Arenicola^ 

 Podarke and several other annelids ; each of the four primary 

 trochoblasts divides twice, the remaining cells forming a more 

 or less complete girdle of 16 cells about the embryo. The 

 completed prototroch, however, differs from that of AmpJiitrite 

 and Arenicola and agrees with that of Podarke, in that, in three 

 quadrants an intergirdle cell is also incorporated in it. In 

 TJialassema, again, the second quartet in each of the a, b and c 

 quadrants contributes three prototrochal cells as in Amphitrite 

 and Arenicola, whereas in Podarke only two are contributed. 



The primary prototroch is completely formed at the 56-cell 

 stage (PL I, Fig. 9). The 16 cells soon begin to swell and 

 elongate to such an extent that at the 64-cell stage the four 

 groups come in contact and form a complete and continuous 

 band around the embryo (PL I, Fig. ii). There is, accord- 

 ingly, no dorsal gap at this stage, but it subsequently arises by 

 the actual shoving apart of the cells at the juncture of the c and 

 d groups at the time of the migration of cells from the first 

 quartet. This formation of an unbroken ring by the primary 

 prototrochal cells is made possible by their superficial position 

 (Text-Fig. 10, A^. In the great majority of annelids, where the 

 prototroch has been accurately determined, the four groups are 

 wedged apart by the dextral cells of the second quartet. An 

 apparent exception has been described for Hydroides and Poly- 

 gordiiis (Wilson, '91), where the protrochal protoblasts divide 

 only once and form a complete band of 8 cells around the 

 embryo ; and also in Nereis (Wilson, '92), where the proto- 

 blasts divide twice, but four of the resulting cells are shoved out 

 and the complete prototroch consists, accordingly, of a ring of 



