BIOL O GICA L LEG TURES. 



FIG. i. Left side of fertilized unsegmented egg of 

 A mphitrite. Polar globules indicate the position 

 of the animal pole. 



is the transverse ciliated fringe, the prototrocJi (Figs. 13-16), 

 and it is the purpose of this paper to bring together and 



compare the observations of 

 several workers who have 

 employed the method of cell- 

 lineage to determine the exact 

 origin of this organ. 



In order to establish a con- 

 crete example for comparison, 

 let us proceed at once to the 

 account of the origin of a 

 simple and fairly typical pro- 

 totroch, namely, that of the 

 marine annelid Amphitrite. 



The spherical egg is of 

 medium size, about one-tenth 

 of a millimeter in diameter, 

 without much yolk, and is 

 unprotected save by a thin 

 wrinkled egg membrane. 

 Fig. i represents the egg 

 seen from the left side. 

 Fertilization has taken place, 

 and the polar globules remain 

 attached. The succeeding 

 figures represent the egg in 

 the same position, unless 

 otherwise stated. Fertiliza- 

 tion takes place in the sea 

 water, and the cleavage pro- 

 gresses so rapidly that the 

 larva swims in a few hours. 

 The first cleavage being un- 

 equal, the 2-cell stage is char- 

 acterized by the possession 

 of a larger and a smaller 

 blastomere, which are represented in side view in Fig. 2. After 

 this the cells divide with some irrregularity in time and in the 



FIG. 2. Left side of Amphitrite egg, 2-cell 

 stage, oriented as in Fig. i. 



