26 



THE ANCESTRY OF VERTEBRATES 



soma 



If we compare the embryo of fig. 5 and 6 with an out- 

 growing trochophora which has already produced a number 

 of segments, the episphere is the part lying in front of the 

 first mesodermic segment and the outgrowing soma is the rest 

 of the body. A difference is that the ectomesoblast seems 

 to have totally disappeared. 



Secondary mesoblast in the prostomium. — In Annelids not 

 only the original mesenchyme of ectodermal origin is found, 

 in the prostomium but also the 

 coelomic trunk mesoderm sends 

 out into it secondary prolongations. 

 This was stated by Hatschek 

 (1886, p. 11) and KLpINENBERG 

 (1886, p. 148) already, and MEYER 

 (1890, p. 299) remarks: "Bei den 

 Anneliden besitzt der Kopflappen 

 keine eigenenMesodermsegmente, 

 sondern erhalt seine peritoneale 

 Auskleidung, wie ich mich uberall 

 davon iiberzeugt habe, durch 

 Ausdehnung der Wandungen des 

 ersten postoralen, also Rumpfso- 

 mitenpaares nach vorn, wodurch 

 die primare Kopfhohle vollstandig 

 verdrangt wird." This latter state- 

 ment, according to ElSIG (1899, 

 p. 230), is not right; in front of 

 the brain, which remains con- 

 nected with the ectoderm, the 

 coelomesoblast cannot penetrate. 

 Thus the antecerebral part of the prostomial cavity preserves 

 its blastocoelic nature and the muscle fibres extended in it 

 are of ectomesoblastic origin. 



Just as in Annelids, the foremost pair of somites in 

 Amphioxus secondarily provides the prostomium with meso- 

 derm, each sending out a forward prolongation into it, 

 known as the rostral or head-prolongation (Kopffortsatz). 

 The anterior end of the notochord in fig. 5 exactly corres- 

 ponds to that of the series of somites; it indicates the 

 limit of prostomium and soma and accordingly lies right 

 under the neurope. Afterwards, however, the notochord, 

 together with the rostral prolongations of the first pair of 

 somites, grows out into the prostomium, providing a firm 



Fig. 6. Horizontal section 

 through the same stage 

 (after Hatschek, 1882, fig. 47). 



