Schakff — On Bohrn's Theories on the Origin of Vertebrates. 29 



is distinguished very materially from the rays of other arches. This 

 mode of origin, indeed, gives us no cause to suppose that the 

 spiracular cartilage consists of a fusion of rays. The cartilage, on 

 the contrary, is much more likely a portion of a visceral arch. 

 Which arch, however, it may have belonged to remains all the 

 more doubtful, as the changes of the anterior arches have been so 

 extraordinary. The adductor mandibulae has been looked upon 

 by comparative anatomists as homodynamous with the adductors 

 of the visceral arches. It is developed from the whole of the walls 

 of the mandibular head- cavities, no portion being separated off as 

 an adductor. The portion lying next the spiracle is differentiated 

 from the other part, and forms the levator maxillae superioris. 

 All the rest goes to form the adductor mandibulae. Hence this 

 adductor cannot be taken as a homologue of the visceral arches. 

 The difficulties connected in deriving the upper and lower jaw 

 from visceral arches are much augmented by the nature of the 

 muscular system — not lessened, as Gegenbaur thinks. 



Dohrn concludes his seventh study with a consideration of the 

 origin of the thyroid gland (see fig. vn.). It makes its appearance 

 early. It arises in the median line just behind the mouth as a 

 solid outgrowth from the endoderm. In time this outgrowth grows 

 towards the heart and obtains the shape of a flask. A formation of 

 follicles then takes place connected with an ingrowth of mesoderm 

 into the body of the gland. Along with the mesoderm, small vessels 

 appear which have been mentioned before. The gland ultimately 

 moves from its original position, travelling more and more back- 

 wards until it occupies the place where we find it in the adult. No 

 doubt, says Dohrn, we have here the last remains of a branchial 

 cleft lying between the hyoid and hyomandibular arches. It 

 might be argued, that as the thyroid arises as a single outgrowth, 

 it cannot represent a pair of gill- clefts. However, as Dohrn 

 pointed out before, bilateral organs often come into such close 

 contact — either vertically or dorsally — as to become apparently 

 unpaired organs. Later on he proposes to discuss the general pro- 

 cesses leading to such fusions of originally bilateral formations. 

 The thyroid artery also points to the fact of a well-developed gill- 

 cleft having once existed in that position. It is also known that 

 during the later stages of development the thyroid gland in many 

 animals becomes paired. According to M. Miiller the thyroid 



