The Pharyngeal Pouches in the Mammalia 



189 



During the progress of this investigation I received much assistance 

 from a number of investigators. To Dr, E. G. Conklin I am greatly 

 indebted for his kind encouragement and helpful suggestions, and for 

 these I desire to express my hearty thanks. To Dr. C. S. Minot I am 

 under special obligations for his kindness in allowing me to examine 

 the fine series of embryos in his charge. I also desire to thank Dr. 

 C. B. Davenport for permission to continue part of this work in the 

 laboratory at Cold Spring Harbor, Long Island. 



I shall present the results of my studies under the following headings : 



I. The Formation and Structure of the Pharnygeal Pouches. 



II. The Later Modifications and Fate of the Pharnygeal Pouches. 



In this study I was enabled to examine the following stages, which 

 I here present in the order of progressive development: 



Pig. 



(3) 6.5 mm., (M^) (1) 



(6) 9.0 mm., (M^) (2) 



(7) 10.0 mm.. No. 401 (4) 



(8) 12.0 mm., " 518 (5) 



(9) 13.5 mm., (M') 



(10) 14.0 mm., No. 65 



(11) 17.0 mm., " 51 



(12) 18.0 mm., (M^) 

 (15) 20.0 mm.. No. 542 



(17) 24.0 mm., " 64 



(18) 25.0 mm., (M^) 



(22) 32.0 mm.. No. 74 



(23) 35.0 mm., (M) 



Cat. 

 4.6 mm., 



6.2 mm., 

 9.7 mm., 

 (14) 10.7 mm., 

 (16) 15.0 mm., 

 (21) 23.0 mm., 

 (24) 31.0 mm., 



No. 398 

 " 413 



" 380 

 " 446 

 " 474 

 " 436 

 466 

 500 



Rabbit. 



(13) 14 days, 10.0 mm.. No. 157 

 • " ~ - 576 



(19) 16* 



(20) 18 



(25) 20 



(26) 21 



17.8 mm., 

 29.0 mm. 



172 



I. The Formation axd Structure of the Pharyngeal Pouches, 



Under this heading I shall describe all stages leading up to the 

 complete formation of the four pairs of pharyngeal pouches character- 

 istic of the mammalian embryo. The stages here considered include 

 Nos. 1 to 4, inclusive. 



The earliest stage of development of the pharynx and its appendages 

 was shown in a cat embryo of 4.6 mm., No. 398 of the Harvard col- 

 lection (Figs. 55 and 56.) The embryo is approximately straight, the 

 headfold is distinctly differentiated, but the posterior two-thirds of 

 the enteric cavity opens widely into the yolk vesicle (at x in the. 

 figures). The neural tube is closed except anteriorly, where a narrow 

 cleft still persists. The optic vesicles are present, but there is no sign 

 of the optic cups. 



