176 DEVELOPMENT OF AMPHIOXUS. 



ectoderm is a prime characteristic of Balanoglossiis and many of 

 the lower worms (e.g. Nemertines) . In none of the craniate 

 Vertebrates is the ectoderm at any time cihated. 



7. (p. 134.) The exact stage at which the club-shaped gland 

 reopens into the pharynx must remain an open question. It is, 

 very probably, subject to a good deal of variation in this respect, 

 occurring now earlier, now later. Experiments to determine the 

 physiological role of this gland are much needed. 



8. (p. 143.) In accordance with Dohrn's conception of the 

 principle of the change of function {Das Princip des Functions- 

 wechscls), the number and nature of the organs of the Vertebrate 

 body, which have been interpreted as modified gill-clefts, are truly 

 astonishing. First and foremost, Dohrn supposed that the Verte- 

 brate mouth arose by the fusion of two gill-slits across the middle 

 line, the old Annehd-mouth, which perforated the central nervous 

 system, having been lost. A great many forcible arguments have 

 been brought forward in support of this hypothesis. Dohrn him- 

 self would probably admit that it is only tenable on his further 

 hypothesis that Amphioxus is a form which has undergone a retro- 

 gressive evolution from the craniate Vertebrates. This was a 

 better hypothesis than that of Semper, who, perceiving that 

 Amphioxus would not fall in with the Annelid-theory, declared, 

 " er sei kein Wirbelthier ; also, auch kein Fisch." 



Besides the mouth, many other structures have similarly been 

 referred back to modified gill-slits, among which may be mentioned 

 the nose, hypophysis, thyroid gland, lens of the eye, and the anus. 

 None of these comparisons is supported by the facts of develop- 

 ment and anatomy of either Amphioxus or the Tunicates, while 

 most of them would appear to be definitely disproved by these 

 facts. 



9. (p. 147.) ■ Since the right metapleural fold bends round to 

 the median ventral line of the snout, as shown in Fig. 38, and 

 since, further, at a later period, the right half of the oral hood is 

 similarly continued round the front end of the body into the 

 dorsal fin, it is clear that the right half of the oral hood must 

 arise essentially in continuity with the right metapleur. On the 

 contrary, the left half of the oral hood arises entirely independently 

 of the left metapleur. It is possible that this discontinuity of 



