54 BIOLOGICAL LECTURES. 



phores characterized by an almost equatorial prototroch, a very 

 large exumbrellar region, and relatively extremely slow devel- 

 opment of the trunk. I merely allude to this here, because 

 in another lecture of this series the causes of equal cleavage 

 in the annelids are considered in detail by Mr. Treadwell. At 

 the other end of the series comes Clepsine with its huge somat- 

 oblasts, in which the parts characteristic of the trochophore 

 are reduced to a mere rudiment, and the entire trunk develops 

 on the surface of the egg. Between these two extremes come 

 the other forms mentioned, and it may be said in general of 

 them, that there is a gradual acceleration in the time of devel- 

 opment of the trunk very nearly in proportion to the increase 

 in relative size of the somatoblasts. Treadwell has elsewhere 

 called attention to part of this series, and has come to the con- 

 clusion that " the extra amount of material stored in cell D of 

 Nereis, AvipJiitrite, etc., is in some way related to the need for 

 an extra amount of somatic and mesoblastic material in the 

 young larva." 



IV. 



But in order to show that the adaptation need not run 

 in the one direction of the trunk, let me cite one more case, 

 which I am able to add through the kindness of Dr. E. B. 

 • Wilson, from some unpublished observations of his on the cell- 

 lineage of a Nemertean. Here the four upper cells of the 

 eight-celled stage are larger than the four surrounding the 

 vegetative pole, the only case of this sort known, I believe. 

 We should expect on a priori grounds that the resulting larva 

 would possess a large helmet in front of the ciliated band, 

 which marks the posterior boundary of the products of the 

 first quartet of ectomeres, with a hardly developed trunk region 

 and a rudimentary archenteron. Such is, in fact, the case. The 

 enormous development in the pilidium of the exumbrella is pre- 

 delineated in the unique third cleavage. 



I do not believe that adaptation is one whit less far-reaching 

 in the cleavage than in the larva. In fact, if the principle 

 which I am defending be correct, the two must be coextensive ; 

 that is to say, adaptation in cleavage is no more caused by 



