224 EDWARD A. BOYDEN 



toads. He was able to show by transplantation methods that 

 the ectoderm of the branchial region and immediately adjacent 

 territory has a certain specificity for building gill filaments not 

 possessed by the remaining ectoderm of the embryo; that a 

 polarity of this ectoderm can be demonstrated; and that even 

 when the entoderm and mesoderm underlying the future gill region 

 in very young embryos are removed the ectoderm alone will pro- 

 duce abortive filaments devoid of blood vessels. It is the ecto- 

 derm of this same region in reptiles and birds which produces 

 rudimentary filaments and they bear at least a superficial resem- 

 blance to some of the abortive structures thus produced experi- 

 mentally in amphibia by Ekman (cf. figs, 26 and 27). In the 

 case of the higher vertebrates the process never passes beyond the 

 initial stages as evidenced by the early appearance of degenera- 

 tion vesicles and the failure of blood vessels to participate in 

 gill formation. 



In the light of Ekman's experiments and the evidence pre- 

 sented in this paper it is doubtful whether the entodermal invagi- 

 nation which Grosser found in the first pharyngeal pouches of 

 young human embryos has been rightly interpreted as an inter- 

 nal rudimentary gill. Grosser recorded his observations as 

 follows : 



A remarkable observation has been made by the author in all young 

 embryos with the first pharyngeal pouches well developed; these are 

 the embryos R. Meyer 335, Hal2 Pfannenstiel III (loaned for this pur- 

 pose), R. Meyer 330, and also a somewhat pathological, young embryo 

 from the collection of R. Meyer. In the re";ion of the first pouch there 

 projects ventrally (figs. 315 and 316) Or caudally (tig. 318j from the 

 closing membrane into the pharyngeal lumen an irregularly knobbed 

 process filled with mesoderm. That it is an accidental structure or 

 due to post-mortem changes seems to be excluded by the regularity of 

 its occurrence (Low has figured, but not described it). It disappears 

 quite early (in the oldest embryo examined, figure 318) it is present 

 only on the left side and is greatly reduced in size; in embryos of 4.25 

 5.0 and 5.8 mm. and in those still older, it is wanting), and may per- 

 haps be interpreted as a rudimentary internal gill^ It would not be 

 the first instance of a very ancient rudiment well developed in the 

 human embryo. Similar structures have not yet been observed in 

 other amniote embryos. (Keibel & Mall Human Embryology, 1912). 



* Italicized by the author of the present paper. 



