10. EMBRYOLOGICAL DEVELOPMENT 119 



up an independent existence it is probably entirely resorbed. Its cells seem 

 to represent nothing more than a stock of yolk-filled endoderm. 



Jagersten (1957) described late embryos of Siboglinum sp. from the 

 Skagerrak. The embryos, found in the maternal tube (three specimens), were 

 very similar to the most advanced embryos described from Oligobrachia 

 dogieli, possessing a separated metasoma with tufts of comparatively large 

 bristles, anterior and posterior ciliated bands and the dorsal ciliated patch on 

 the mesosoma, but still lacking any tentacular rudiment. Besides the large 

 bristles of the metasoma Jagersten also noticed very small lateral bristles in 

 the mesosomal region and behind the posterior metasomal ciliated band 

 (Fig. 83). Such bristles have not been found in the embryos of Siboglinum 

 caulleryi and Oligobrachia dogieli. 



Without sufficient foundation Jagersten regarded the posterior divided 

 portion as conjoined protosoma and mesosoma. He thus misorientated the 

 body of the embryo, regarding as the front end that which is in fact the 

 posterior end. If a tentacle rudiment had been present he would of course 

 have avoided this mistake. He also decided that the ciliated patch in the 

 middle of the body was ventral — a decision linked with his concept of the 

 determination of the dorsal and ventral surfaces of the body of the adult 

 (p. 8). And finally, Jagersten described a small indentation at the tip of the 

 metasoma [the front end of the embryo in his view] and interpreted it as the 

 rudiment of a stomodaeum. Its location at the hind end of the body, not joined 

 to the endodermal mass in any way, excludes the possibility of any such con- 

 struction. 



Possible changes of organization during the 

 postembryonic period 



The fully formed young animal probably makes its way out of the maternal 

 tube without any assistance from the mother and immediately begins to 

 make its own little tube. There must be sufficient food reserves still stored in 

 the cells of the endoderm to enable it to form the tentacular apparatus and so 

 to begin feeding. 



The structure of the young animal soon after emergence from the maternal 

 tube is unknown. One can readily imagine, however, the changes in organiza- 

 tion which are happening at this age. The mesosomal bridle is being differ- 

 entiated, the metasoma is growing rapidly in length, all the definitive organs 

 are formed, including the dorsal ciliated band, the adhesive papilla and the 

 girdles of toothed platelets. At the same time many internal organs make their 

 first appearance — the multicellular flask-shaped glands, the nervous system, 

 many of the blood vessels, the heart, coelomoducts and reproductive system. 



