594 HAROLD HEATH, 



has probably the same origin as in Ischuochiion and, as will be shown, 

 in the Annelids. 



The Annelid prototroch consists of a simple band of cells en- 

 circling the embryo, composed of relatively few cells whose origin 

 has been determined in several cases. There is a general correspond- 

 ence throughout, but some modifications have been described whereby 

 the development departs from the general i)]an. For example, Wil- 

 son shows that the prototroch of Nereis arises wholly from the pri- 

 mary trochoblasts, 12 of the 16 cells from this source becoming 

 functional. The second quartette accordingly contributes nothing to 

 this structure, although some of its products become post-trochal cells 

 lying below the locomotor organ. This prototroch, and possibly that 

 of Ilydroides, in which there are 8 cells in the prototroch (Wh.son), 

 is much simpler than is ordinarily found in Annelids. And as has 

 been noted, all its cells arise from the primary trochoblasts and 

 therefore from the same cells which enter the velum in Ischiochiton. 



In Ampliitriie and Clymenella Mead shows that the primary 

 trochoblasts give rise to 16 cells all of which enter the functional 

 prototroch, and the same fact has been proved to be true in Podarke 

 (Treadwell) and in Arenicola (Child). The origin, development 

 and fate of these cells is precisely similar to the pri- 

 mary trochoblasts in Ischiochiton. 



The second quartette in Amphitrife, Cb/menella and Arenicola 

 furnishes three cells in each quadrant except the posterior, which 

 enter the prototroch. Two of the three are homologues of the divided 

 tip cell in Ischiochiton while the third corresponds to a post-trochal 

 cell. It is to be noted however that while the second quartette sup- 

 plies a cell which is non-functional in Ischuochiion, this latter form 

 furnishes an accessory trochoblast whose homologue is non-functional 

 in the Annelids. The remarkable fact now presents itself, twenty- 

 two of the twenty-five prototrochal cells in Amphiirite, 

 Clymenella, Arenicola and possibly Podarke arc exactly homo- 

 logous with the velar cells in Ischnochiton. A more complete 

 resemblance it would be difficult to find and I see no other alter- 

 native than that it is an indication of a close genetic relationship 

 between the Molluscs and the Annelids. 



2. Ectoblastic Cross (Molluscan Cross), 

 a) Its Formation. Briefly recapitulating, the first division of 

 the first quartette of ectomeres gives rise to the primary trochoblasts ; 



