Jones— Nestling Feathers. 7 



ion, and this layer remains single. The dividing mucous cells, on the 

 contrary, rapidly form two or more layers, or a confused mass of cells bounded 

 by a well marked layer next to the derma (PL I, Fig. 7). 



In the further growth of the down papilla the cells on the anterior 

 surface divide more rapidly than those on the posterior surface, so that 

 the whole papilla is turned sharply backward (PI. I, Fig. i), finally 

 coming to lie almost parallel to the skin surface. No sooner has this papilla 

 taken a definite turn backward than cell division within it ceases except in 

 a narrow region at its base just outside the skin surface; consequently all 

 further growth of the papilla results from this narrow region of division and 

 from the growth of the individual cells which are supplied with nutriment 

 by numerous blood capillaries in the pulp. 



As long as the down papilla continues to elongate it continues to be 

 an evagination of the epidermis with a core of dermal tissue. When the 

 papilla has attained its full length, save for some further growth of its 

 individual cells, the continued cell division results in an invagination of the 

 mucous layer of the feather-germ at an angle of about 160° with the down 

 papilla. At first this invagination is pushed farther in by the division of 

 the cells at the skin surface, but the cell division soon changes its position 

 to the proximal end of the invagination, and henceforth all growth proceeds 

 from this proximal end by cell division there. This invagination is the be- 

 ginning of the definitive feather (PI. I, Fig. i, grm. df). The epitrichial 

 layer does not participate in this invagination, and therefore does not contrib- 

 ute to the structure of the definitive feather. 



My investigations have shown that the invagination of the feather 

 germ, which represents the beginning of the definitive feather, begins about 

 the 1 2th day of incubation of the chick, of the duck, of the terns, and of the 

 robin. My observations also indicate that with these birds differentiation of 

 the definitive feather elements proceeds slowly until about the 17th day, 

 after which time the definitive feathers develop rapidly. During the interval 

 between the 12th and 17th days the feather rudiment seems to be engaged 

 chiefly in the formation of the feather follicle. 



At the base of the down we may distinguish three sorts of cells, 

 following Davies' classification. The epitrichial cells, consisting of a single 

 outer layer (PI. I, Fig. 6, Ep), an inner layer of mucous cells cubical in 

 form and more or less regular (PI. I, Fig. 6, cl. cyl), and a mass of more 

 or less irregular cells between these two layers (PI. I, Fig. 6, cl. im). The 

 innermost layer may be distinguished as the "cylinder cell layer" (cl. cyl), 

 and the irregular cells as the "intermediate cells" (cl. im). These two lay- 

 ers are derived from the mucous cells by division. 



I can add little to Davies' account of the early stages in the development 

 of the down. I submit Figures i, 6, 7, 8, and 9, as amplifications of 

 those given by him. His work was mostly done upon pigeon and canary 

 embryos, while these figures are from a chick embryo. I have also studied 



