596 HAROLD HEATH, 



responding one passing through the right and left halves of the body. 

 The tip cells lie in these axes but the major portion of the arms lies 

 to the right in each quadrant. But later the radial condition is as- 

 sumed and the arms become anterior, posterior, right and left, which 

 position they probably permanently retain. 



c) Its Occurrence in otherForms. Among the few Gastro- 

 pods which have been accurately studied a cross is shown to be pre- 

 sent and in general appearance, position and history, as far as these 

 have been traced, it is similar to the Chiton cross, and yet the two 

 structures are not homologous. In what does the essential difference 

 consist? In attempting to answer this question it may be well to note 

 that Ischnochiton throughout is larval development shows a more 

 primitive character than the Annelids or Gastropods whose cell lineage 

 has been carefully studied, and that its development is more 

 closely related to the Annelids than to the Gastropods. 

 Hence in considering the differences that exist between the Gastro- 

 pod and Chiton crosses we shall note first the condition of affairs in 

 the Annelids and in this light consider what obtains in the Gastro- 

 pods, and then if possible bring the various structures into harmony 

 with one another. 



In the first place it should be borne in mind that in Jschnoehiton 

 there are three divisions of the first quartette before the basal cell 

 is formed, and that this latter cell is formed by a leio tropic di- 

 vision in which the accessory trochoblast is also formed. 



In Amphitrite, as shown by Mead, the first division of the first 

 quartette of ectomeres gives rise to the trochoblasts. These cells are 

 relatively larger than those in Chiton, appearing fully as large as the 

 parent cells. The second division forms the apicals which are also 

 relatively larger than those we find in Ischnochiton. Hence between 

 the formation of the trochoblasts and the apicals the 4 remaining 

 cells, called in Annelids the intermediate girdle cells, are the smallest 

 of the first quartette. This is clearly brought out in diagram C, c, 

 and also by comparing it with the same stage in Ischnochiton (C, a). 

 In the diagram the trochoblasts have divided and either product is 

 nearly or fully as large as the intermediate cell, and this latter is in- 

 variably smaller than the apical cell of the same quadrant. In Ischno- 

 chiton on the other hand either of the two trochoblasts in each 

 quadrant is about the same size as an apical cell, and the cell with 

 the same designation as the intermediate cell is nearly or quite as 

 large as the two trochoblasts and the apical taken together. But 



