206 EDWAED A. BOYDEN 



be brought into line with the gill filaments of amphibians and 

 fishes. Subsequent study of older and younger chick embryos, 

 together with the finding of similar structures in reptilian em- 

 bryos, seems to warrant such a conclusion and the presentation 

 of this material from a phylogenetic standpoint. 



ORIGIN AND EARLY DIFFERENTIATION OF GILL-FILAMEXTS IN 



THE CHICK 



The history of these filaments, covering a period from the 

 beginning of the fourth to the middle of the eighth day, occupies 

 nearly one-fifth of the total period of incubation. Throughout 

 this time the epithelium of the filaments themselves as well as 

 the branchial epithelium which gives rise to them is character- 

 ized by the presence of what appear to be degeneration resides. 

 These accompany, and thus may be said to register, an activity 

 of the epithelium of which the filaments seem to be the fruition. 

 They first appear in the entoderm, but later arise in the adjacent 

 ectoderm, where they come to exist in greatest numbers. As early 

 as the seventy-six hour stage 'Harvard Embryological Collec- 

 tion, Series 2057, 6.8 mm., 76 hours; and Ser. 1953, 8.0 mm., 78 

 hours) the first of them may be seen in the posterior or postero- 

 medial walls of the first four pharyngeal pouches at a time when 

 the first three gill clefts have broken through and the fourth pouch 

 touches the ectoderm (figs. 4 and 10). WTien complete, each 

 vesicle is a clear spherical cyst, embedded in the epithelium, meas- 

 uring some twelve to twenty microns in diameter, that is from 

 two to three times the size of the erythroblasts in neighboring 

 blood vessels. Each is surrounded by a wall of its own, com- 

 parable in thickness to a nuclear membrane, the whole enclosing 

 pycnotic nuclei and cytoplasmic fragments in different stages of 

 degeneration. Favorable sections, such as figure 25, indicate 

 that these cysts result from the nearly simultaneous disintegra- 

 tion of adjacent epithelial cells. In places where two or more 

 cysts have coalesced, the resulting structure often bears a super- 

 ficial resemblance to capillaries containing corpuscles in the un- 

 derlying mesenchyma, but the most careful search fails to re- 

 veal any direci relation between the two. Of interest, in con- 



