550 CKESSWELL SllEAKEli. 



lip a bilateral position. In Tlialassenia it arises from the 

 first and third qnartettes. In soiue thirty Annelids it can be 

 said definitely that the eoelomesoblast arises from the posterior 

 cell of the fourth quartette, while the larval mesoblast arises 

 from the first and third. This title of "larval niesoblast" does 

 not mean necessarily that it is contiiied alone to the organisa- 

 tion of the larva, for the greater part of it enters into the 

 structure of the adult. The same has been shown to be the 

 case in a number of other Annelids, as in Polygordius, 

 Podarke, and Thalassema. 



Meyer long ago put forward the theory that the mesen- 

 chyme of higher forms corresponds with the mesoderm of the 

 lower ; that the larval mesoblast of Annelids and Molluscs is 

 to be homologised with the adult mesoderm of Platodes. 

 AVilson (50) has shown in Leptoplana that the mesoderm 

 in this Polyclad arises from the second and tliii'd quartettes, 

 while in Annelids the larval mesoblast, as I have mentioned 

 ■Jibove, takes its origin from the same quartettes. He has 

 established that here the large cell 4d is almost entirely 

 entoblastic. The early development of the Polyclad Plano- 

 cera has been studied by Surface (40) : "At the forty-four 

 cell stage the posterior cell of the fourth quartette {4d) buds 

 a single large cell into the interior of the embryo; both of 

 these subsequently divide bilaterally (Text-fig-. 3). Of these 

 four cells the two upper and inner (Text-fig. S, 2d) give rise 

 to a portion of the niesoderu), and possibly a small part of 

 the endoderm (Text-figs. 1—3, Jfd). The lower pair lying on 

 the surface of the embryo give rise practically to all the 

 endodermal part of the alimentary canal." Tims the history 

 of this cell {4d) in this Polyclad shows a remarkable resem- 

 blance to its homologue in Molluscs and Annelids. "A 

 portion of the mesoderm, chiefly that part lying round the 

 })harynx, is derived from cells of the second quartette, and 

 thus corresponds with the secondar}^ mesoblast or larval 

 mesenchyme of Annelids and Molluscs (Texl-(ig. 1, 2h). In 

 the spiral cleavage the segregation of the ectoblast in three 

 quartettes, the formation of a large portion of the mesoderm 



