612 HAROLD HEATH, 



cleavages are similar throughout but there is a noticeable difierence 

 in the size of the first somatoblast products as compared with cor- 

 responding cells of the remaining quadrants. Among other Annelids, 

 such as Nereis, Amphifrite and Arenicola, the effect of the bilateral 

 form of the larva is more pronounced, manifesting itself at the first 

 cleavage. The same is also true of the Lamellibranch Unio. The 

 somatoblast in these cases is characterised by a relatively large size 

 and the typical radial symmetry of this quartette is proportionately 

 disturbed. This divergence becomes more marked as the cells 

 commence to develop when it is very difficult, if not impossible, to 

 homologize the cleavages of the first somatoblast and the other 

 quadrants. A similar difficulty is also experienced when an attempt 

 is made to homologize first somatoblast cleavages in embryos of two 

 animals in different classes and it is not much lessened even in a 

 comparison of embryos in the same class. 



The difficulty is not so great if one attempts to homologize the 

 remaining second quartette cells and it appears probable as mentioned 

 above that in the original ancestor this quartette underwent a similar 

 development' in all quadrants. A secondary shifting of the axes has 

 produced secondary changes in 2 d^ but it by no means necessarily 

 follows that such secondary effects should manifest themselves in 

 exactly the same fashion in different classes or even orders, indeed it 

 would be strange if they did. 



Finally, Mead has made the interesting discovery in Amphitrite 

 that with the exception of small areas in the immediate vicinity of 

 the mouth the entire trunk forms from 2ä. In Chiton the trunk in 

 its early stages is almost as extensive as in this Annelid but at least 

 one half of it is formed from the third quartette which in Annelids 

 remains of small size. 



VII. History of the Third Quartette. 



1. Early Cleavages and Later History. 

 The history of the third quartette is the simplest and least varied 

 in the development of the ectoblast. At its first division it produces 

 the third quartette stomatoblasts whose fate is considered on page 617. 

 To the remaining products attention will here be directed. Their 

 first division is meridional, the spindle being horizontal, and as a 

 result two cells of the same size are produced in each quadrant. In 

 this and the succeeding cleavages of this quartette the posterior 



