248 



BIOLOGICAL LECTURES. 



genetic form of cleavage, which finally attains its 'most pro- 

 nounced expression in teloblastic growth. 



A comparative study of the cell lineage of various Poly- 

 chaeta shows a considerable degree of difference within the 

 limits of this group as regards the stage at which the spiral 

 form of cleavage disappears (cf. Figs. 7, 19, 20), but it is 

 impossible to discuss this point in detail here. 



2. Cases of what maybe called "reversion" to the spiral 

 type of cleavage occur, i.e., cells which have departed from the 

 spiral type may return to it in later stages for one or more gen- 

 erations. Numerous examples of this " reversion " occur. I 



FIG. 19. 



FIG. 20. 



FIG. *<). P0darke. Cross formed by spiral divisions (after Treadwell). 



FIG. 20. Chcetopterus. Cross does not appear because of extreme spiral character of divisions 

 which commonly give rise to it (after Mead). 



wish to mention here some of my own observations on Areni- 

 cola without reference to other forms, for, where special at- 

 tention has not been paid to this point, figures and general 

 statements are usually too indefinite to serve in this connec- 

 tion. The cross in Arenicola is formed by radially symmetrical 

 divisions (Fig. 7). In the division following this the four cells 

 terminating the arms of the cross often show traces of spiral 

 divisions. This is especially noticeable in the ventral arms of 

 the cross (Fig. 8, i# 1122 , i" 22 ), which often curve at the tip 

 toward the right side of the egg. Both the spindle and the 

 position of the cells after division indicate in many cases the 

 spiral nature of the cleavage. The direction is Iseotropic, as it 



