634 HAROLD HEATH, 



studied. In addition, Conklin claimed that the primary trochoblasts 

 of Molluscs are, at least in part, precisely similar in origin and 

 destiny to those of Annelids , that the axial relations of all the blasto- 

 meres with the possible exception of the macromeres are the same in 

 both these groups of animals, and finally the larval mesoblast of 

 Crepidula arises from the same group of ectoderm cells as in Unio. 



The above lists consist of no less than twelve points wherein there 

 is an essential similarity in the development of Annelids and Molluscs, 

 and the development of IschnocMton with its more direct and less 

 involved development serves in the most emphatic manner to prove 

 the correctness of these observations. 



In addition to the above resemblances, I would once more em- 

 phasize the wonderful homology which exists between the Annelid and 

 IschnocMton prototroch, in which 22 of the 25 cells in the former 

 have exactly the same origin, direction of cleavages and destiny as 

 in IschnocMton, and the remainder of the first quartette, forming the 

 head vesicle, with its rosette series and Molluscan cross cells or inter- 

 mediate girdle cells, has in all probability the same fate in both. 



The second quartette has been shown to have exactly the same 

 origin and relation in both classes and several of the early divisions 

 are similar. In some forms precocious segregation has produced ex- 

 cessive modification in 2d, so great that it appears at first sight 

 almost impossible to reconcile the differences that have arisen; but 

 a more careful study shows conclusively that they are but variations 

 of a common radial type, and from this standpoint it is possible to 

 trace and understand a remarkable series of resemblances that appear 

 otherwise inexplicable. 



In the invagination modifications have likewise arisen. Ac- 

 cumulation of yolk and precocious segregation have both been in- 

 strumental in producing many changes, but whether the gastrula be 

 produced by epibole or embole the closest resemblances appear. In 

 all cases the macromeres and three of the fourth quartette products 

 produce the mesenteron, and the relation of these cells and their 

 behavior during gastrulation are very similar. Also the mesoblast in 

 its position and development exhibits fundamental resemblances 

 throughout. The blastopore is usually quadrangular with second 

 quartette cells in the angles and third quartette forming the sides, 

 and during gastrulation some of these second quartette products 

 become invaginated in Nereis and IschnocMton and enter into the 

 formation of the stomodaeum ; the same thing is probably true of the 



