ORIGIN AND STRUCTURE OF THE HEAD 



25 



as derived from the primary entoderm. The contrast between 

 the "primary mesoderm or embryonal mesenchyme" and 

 the "secondary or coelomatic mesoderm" was first empha- 

 sized by Meyer (1890) who compared the former to the 

 mesenchyme of plathelminthes and derived the coelomic 

 pouches from the genital follicles of the latter (gonocoel- 

 theory). These follicles often even already exhibit a tendency 

 to regular metameric arrangement. According to this view 

 we may consider the ectodermal mesenchyme of the trocho- 

 phora as a last remnant of that of mesenchymatous worms, of 

 their larva, the protrochula(HATSCHEK) — MuLLER's larva of 

 Polyclads, pilidium and Desor's larva of Nemerteans— and 

 of the Ctenophores, in the same way as the protonephridial 

 head-kidney found in several trochophoras reminds us of 

 the richly branched excretory system of ancestors like the 

 mesenchymatous worms or the Rotatoria. 



Praeoral lobe in Amphioxus. — The mouth belongs to the 

 first trunk segment, the peristomium. It is situated just 

 behind the limit of the prostomium and the first segment. 

 Now, if we compare this with what we find in a young 

 embryonic stage of Amphioxus, as represented in fig. 5 and 



6, there is a remark- 

 prost. I soma able agreement to 



be noticed. In front 

 of the first pair of 

 coelomic pouches 

 we find here also a 

 part of the body in 

 which originally no 

 coelomic mesoderm, 

 and even no meso- 

 derm at all, is pre- 

 sent, and which we 

 can compare to the 

 praeoral lobe or 

 prostomium of the 

 Annelids. This supposition is confirmed by the exactly 

 corresponding situation of the mouth in the two cases. 

 In Amphioxus of course the old mouth, the neuropore, is 

 meant. Just like the mouth in Annelids, it is found at the 

 limit of the cephalic lobe and the first segment, which 

 is here indicated by the first pair of coelomic pouches. 

 The agreement so far hardly could be more complete. 



Fig. 5. Optic longitudinal section of a 

 young stage of Amphioxus 

 (after Hatschek, 1882, fig. 46). 



