44 



BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



Development of larva. Segmentation of the fertilized ovum is complete and 

 nearly regular (see fig. 2). A blastula in the shape of a biconvex lens is formed 

 and in its interior or blastocoele the primitive endoderm cells (end) become visible. 

 These increase by division forming free cells which almost fill the blastocoele and 

 represent probably both the endoderm and mesoderm. Small cavities occur in 



disk 



nternal sack 



FIG. 2. Early Stages in the Development of the Chellostomata. 



See text for description of the figures. 



A-G. Drawings illustrating the development of Buyiiln. (After Vigelius. ) 



cent, central mass of cells: cur, corona; cct, ectoderm: end, endoderm; x<-ii. segmentation 

 cavity. 



H. Longitudinal section of the attached larva of Bitgulu. 



o, cells of corona ; r, rudiment of the zooid in the form of a sac ; s, basal plate of everted 

 sucker ; v, vestibule. 



/. Larva of Bugula plumosa, highly magnified. 



<7. Sagittal section of Bugula (H-.T after B.-irroisi. 



cent, central tissue; cor, corona; disk, retractile disk: c, ectodermal groove; int. sac, internal 

 sack; p, pyriform organ; pall, pallia!. groove. 



this mass of cells and form the primitive coelome by their union. Next the equa- 

 torial region of the embryo becomes thickened and provided with cilia forming the 

 r-orona (fig. 2, G. cor.). On the oral side of the corona the circular pallial 

 groove arises. The sucker, a structure which afterwards serves to fix the larva, 

 then develops as a sac-like invagination of the ectoderm or what later becomes the 

 oral side of the ciliated ridge. On the oral side near the corona a second depression 



