2l6 



HISTOLOGY 



ectodermal, but before they develop the oral plate has disappeared and 



the boundary between ectoderm and entoderm cannot be sharply drawn. 

 The entoderm of the mouth and pharynx is a layer of epithelium lining 



a broad, dorso-ventrally flattened cavity. From this cavity, a succession 

 of paired outpocketings grow out laterally to meet 

 the ectoderm on the side of the neck; these are 

 the pharyngeal pouches. They reach the ectoderm 

 at the bottom of furrows or clefts, corresponding 

 m num ^ er wit* 1 t* 16 pharyngeal pouches, and there 

 the two germ. layers fuse. The plates thus formed 



FIG. 203. DIAGRAM SHOWING are comparable with the oral plate, and in fishes 



THE RELATIONS BETWEEN 



ECTODERM AND ENTO- they rupture producing the branchial clefts (gill 



DERM IN THE MOUTH OF A J 

 MAMMALIAN EMBRYO. 



clefts). 



Their arrangement in a young dog-fish is shown in Fig. 



1., and p. 1., Anterior and 

 posterior _ lobes of the 

 hypophysis; m. t., medul- 

 lary tube; ph., pharynx; _, , . . . , , 



o. p., oral plate; x. and y., 204. The mouth, m, leads into a cavity, the pharynx, which 

 opens freely on the outer surface of the fish through five 

 gill clefts, g.c. It also opens to the surface through the 

 spiracle, sp., a structure similar to the gill clefts, but an- 

 terior to them and having a more dorsal aperture. In respiration water is taken in 

 through the mouth and spiracle, and passes out through the gill clefts; but sometimes 

 water is ejected through the spiracle. In mammals the corresponding structure is 

 counted as the first gill cleft. 



In mammalian embryos there are four well-defined pharyngeal 

 pouches on either side, which reach the ectoderm at the bottom of corre- 

 sponding grooves; but if their closing plates ever rupture they are soon 

 restored, and permanent openings from the pharynx on the side of the 



FIG. 204. HEAD OF A YOUNG DOG-FISH. 

 g. c., Gill cleft; m. lk ,mouth; n., nasal pit; sp., spiracle. 



FIG. 205. HEAD OF HUMAN EMBRYO OF 

 10 MM. 



c. s., cervical sinus; g. c. 2., second branchial 

 groove; h., hyoid arch; m., mouth; md., 

 mandibular process; n., nasal pit; sp., 

 auditory (spiracular) groove. 



neck are not found. The first pouch, corresponding with the spiracle, 

 connects with the auditory groove (Fig. 205, sp}. Around it the external 

 ear develops, so that its position is always evident. The ectodermal 

 depression which connects with the second pouch disappears, except in 

 rare cases, where it forms a cervical fistula. This is a pit, or slender tube, 

 in the skin of the neck, situated primarily between the hyoid bone and 

 thyreoid cartilage. The third and fourth pouches connect with the 



