636 HAROLD HEATH, 



is soon superceded by the permanent bilateral form. In other cases 

 where the precocious segregation is not manifested in the early stages 

 a complete radial symmetry exists which ultimately becomes modified 

 into the bilateral type, but at a much later period than in the 

 first case. 



Ischnochiton presents a remarkable example of this latter type: 

 its quadriradial symmetry is in the highest degree apparent. Three 

 quartettes of ectomeres arise in as many cleavages of the macromeres, 

 and the structures to which they give rise, the Annelid and Gastropod 

 crosses, the three species of trochoblasts , the groups of second and 

 third quartette products, all for a considerable period conform to the 

 radial type. Also the entomeres and macromeres, scarcely less perfect 

 in this regard, aid in the formation of a beautifully symmetrical 

 embryo in which all quadrants are essentially identical. 



The first changes which creep in relative to the shifting of the 

 embryonic axes are due wholly to accelerated cleavages. We have 

 already seen that this early effects deep seated modifications in the 

 first somatoblast, but in the other ectomeres the change is much more 

 gradual. For example, the acceleration in the divisions of the 

 posterior third quartette cells is small at first, and it is not until 

 there are over 200 cells in the embryo that the diiference between the 

 groups becomes strikingly apparent. And as regards the cells in the 

 anterior hemisphere , a glance at Fig. 34 will show that the radial 

 symmetry is but slowly giving way to the bilateral. 



In Nereis and Amphitrite, on the other hand, the bilateral con- 

 dition has become reflected backward onto the early stages to such 

 an extent that it affects the first division, and later strictly bilateral 

 cleavages appear in various groups of cells early in the development. 

 But in the case of Lepidonotus and Ischnochiton this is by no means 

 so apparent, and it is only by the aid of certain landmarks that one 

 is able to orient the embryo up to the 60 cell stage, and in Ischno- 

 chiton as we have seen the radial condition becomes but slowly modified 

 into the bilateral. 



The question now arises, what does this early radial condition 

 represent? A group of cells of no morphological value or of the 

 utmost fundamental importance in the formation of the embryo? 

 I believe many facts in the development of Ischnochiton and other 

 forms lead to the belief that the truth lies on the side of the latter 

 proposition; that the original ancestral form of the trocho- 



