1007 



shaped gland (van Wi.the, 1893, p. 153, cf. also His, 1887, p. 429); 

 they represent the second pair of gill-slits. The first pair of gill- 

 pouches, at the limit of prostomium and first segment, is represented 

 by Ihe bilaterally symmetrical "anterior entoderm pockets" or "head- 

 cavities" (Hatschek, 1892, p. 144) of which only the left one still 

 gets an opening, known as Hatschek's pit, to the exterior ; the right 

 one does not open, but gives rise to the so-called praeoral coelome. 



To each somite a dorsal nerve belongs, to the first one, however, 

 two, situated close to each otlier and compared by Hatschek 

 (1892) to the two parts of the trigeminus in Craniotes, which com- 

 monly is considered as a double nerve, by me, however, with 

 Balfour (1878, p. 214) as a single one which sometimes may be 

 split into two (cf. facialis and acusticus) and belonging to the first 

 or mandibular segment (v. infra). The same holds for both the 

 anterior spinal nerves of Amphioxiis, which accordingly 1 designate 

 together as no 1. No distinction can be made as yet between cranial 

 and trunk nerves, dorsal and ventral roots remain separated along 

 the whole body. Here already, however, the fourth nerve (Hatschek's 

 5^^, 1892, p. 143), the future vagus, is distinguished by its strong 

 development and it is especially this nerve which communicates 

 with the longitudinal plexus supplying the gills (ramus branchio- 

 intestinalis vagi of Craniotes). Thus the first four somatic segments 

 evidently correspond to the trigeminus-, the acustico-facialis-, the 

 glossopharyngeus- and th.e vagus-segment of Craniotes (v. infra). 



The prostomium does not contain any mesoderm of itself, its 

 mesoderm is derived from the first somite, as is also found ver}' 

 generally in Annelids (cf. e.g. Meyer, 1890, p. 299). The proper, 

 ectodermal, so-called primary or larval, mesenchyme (Conklin, 1897, 

 p. 151) of the prostomium of Annelids, a last remnant of the mesen- 

 chyme of the primary body-cavity of flatworms (Meyer, 1890), has 

 evidently disappeared in Vertebrates, together with the so-called 

 head-kidney of the trochophora, a last rest of the protonephridia of 

 flat-worms which, however, have been preserved in certain Annelids 

 and in Amphioxus (Goodrich, 1902). 



The asymmetrical origin of the mouth of Ampliioxits gives us the 

 key to the interpretation of the larval asymmetry. 



Petromyzon is distinguished from Amphioxus by the possession 

 of a praechordal brain, the palaeocranium of Kupffek, which 

 together with the inverted eyes has arisen from the dorsal half of 

 the surface of the prostomium, so that the neuropore, also in 

 the other Craniotes, is not situated any longer dorsally, and 

 above the anterior end of the notochord, but terminally, far in 



70* 



