Scharff — On Dohrn' s Theories on the Origin of Vertebrates. 21 



proves that the external cartilages have nothing to do with the 

 arches of Petromyzon, which are true arches. (See fig. vi.) 



The commencement of the real gill-formation takes place as 

 soon as the first four clefts and the mouth have been formed. The 

 branchial lamellae originate as semicircular evaginations of the 

 external epithelial layer. 



As they are further differentiated, however, a great difference 

 appears between the anterior and posterior rows of lamellae of each 

 arch. While those of the anterior rows gradually assume the form 

 of true lamellae, those of the posterior rows elongate considerably 

 and project through the gill-clefts. The latter constitute the so- 

 called branchial filaments of the embryonic elasmobranch. As 

 regards their function, Dohrn suggests that they absorb yolk from 

 the surrounding medium. He found that the filaments as well as 

 the veins were filled with an emulsion of yolk, but he never met 

 with any in the branchial arteries or the heart. 



Dohrn next goes on to the consideration of the thymus in 

 elasmobranchs, discussing its origin and function. 



Some time previous to the differentiation of the branchial rays 

 five buds appear one after another in the upper angle of the five 

 gill-clefts. The last of these soon disappears again. They are 

 epithelial in their origin, only the lower layer of the epithelium 

 taking part in their growth ; but from the very commencement, 

 mesodermic cells as well, seem to enter into the formation of the 

 buds. After having increased considerably in size, the buds sink 

 into the mesoderm. The cause of their disappearance from the 

 surface is to be sought in the reduction in size of the visceral clefts 

 referred to above. 



That these buds represent the thymus of elasmobranchs is 

 evident from their position. Dohrn concludes this article by refer- 

 ring to the function attributed to the thymus by the various 

 authors. 



Y. — The Origin and Differentiation of the Visceral Arches in 

 Petromyzon. 1 



We know, from the researches of Scott and Balfour, that 

 the visceral arches arise in the shape of head-cavities, lying 



I Ibid., vol. v. 1884, pp. 151-160. 



