THOCHOPHOKE OF HYDROIDES UNCINATUS (ErPOMATl's). 551 



fro:n 4d, the formation of the apical cells, and in many other 

 details, the development corresponds to that of Annelids and 

 hio'her forms/' 



In Platodes the mesoderm has a radial origin, and this is 

 also tlie case in a number of Annelids with regard to the 

 larval mesoblast. I have also mentioned in a nuudjer of 

 Annelids and Molluscs that a small portion of 4'^ i« ento- 

 blastic. The condition in Polyclads, whei-e the greater part 

 of 4^d is entoblastic, is suggestive of a more primitive condi- 

 tion than that found in Annelids. If the germ-cells in the 

 Polyclad arise from the ^(? portion of the mesoderm, then 

 the homology of this cell with the cell 4d of Annelids Avould 

 be complete. The history of the posterior cell of the fourth 

 quartette in Polyclads, Annelids, Lamellibranchs, and Gastro- 

 pods has a remarkable resemblance in all these forms, and the 

 relation it shows with the endoderm of the gut points clearly, 

 as Wilson (50) has said, to the way in which teloblasts have 

 arisen by progressive specialisation from a purely endodermio 

 origin of the coelomesoblast as retained in an unaltered con- 

 dition in the Echinodermata to-day. As he says, it is difficult 

 to explain these facts otherwise than on the grounds " that 

 cell outlines represent definite boundaries of differentiation 

 areas in the developing embryo." 



Child (6), on the contrary, claims that no importance can 

 be attached to resemblances of this nature, and that in the 

 case of the cell 4d they are purely coenogenetic, and have to 

 do with the formation of the larval body from a growing 

 region at its posterior extremity, and the resulting segrega- 

 tion of material at this point. 1 think this cannot be said of 

 all cases where there is a similar segregation of the cceIo- 

 mesoblast. The growth of the adult from the Glochidium 

 larva is diiferent in man}^ respects from the growth of the 

 adult worm from the trochophore, yet in both we get a marked 

 segregation of the coelomesoblast. 



With the high degree of specialisation shown by eggs that 

 give rise to a free-existing larva, the coelomesoblast, which 

 primitively arose as diverticula from the gut, became restricted 



