114 



BIOLOGICAL LECTURES. 



Fig. I. — Left side of fertilized unsegmented egg of 

 Amphitrite. 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 AmpJiitrite. 



The spherical &^'g is of 

 medium size, about one-tenth 

 of a millimeter in diameter, 

 without much yolk, and is 

 unprotected save by a thin 

 wrinkled Q%% membrane. 

 Fig. I represents the &g^ 

 seen from the left side. 

 Fertilization has taken place, 

 and the polar globules remain 

 attached. The succeeding 

 figures represent the q^^ 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 A niphiirite egg, 2-cell 

 stage, oriented as in Fig. i. 



