THE ORGANS OF THE INNER GERM-LAYER. 



289 



From the fourth week of development onward the visceral arches 

 begin to be displaced in relation to one another, owing to a more rapid 

 growth of the first two than of the following ones (fig. 156). " They 

 glide over one another," as His remarks, " like the tubes of a telescope, 

 in such a way that, viewed from the outside, first the fourth arch is 

 surrounded and covered in by the third, and this in turn by the 

 second, whereas on the inner surface, that which is turned toward the 

 pharynx, the fourth arch 

 lies over the third, the 

 third over the second." As 

 a result the length of the 

 oro-pharyngeal cavity is 

 relatively less in the older 

 than in the younger em- 

 bryos. In consequence of 

 this unequal growth, which 

 moreover takes place in an 

 entirely similar way in the 

 embryos of Birds and Mam- 

 mals, there js formed a deep 

 depression of the surface at 

 the posterior margin of the 

 cephalo-cervical region, the 

 neck-sinus, sinus cervicalis 

 (RABL) or sinus prcecervi- 

 calis (His) (figs. 156 and 

 158 hb). In the depths of 

 this depression and on its 

 front wall lie the third 

 and fourth visceral arches, 

 which are now no longer 

 visible from without. The 

 entrance to the sinus is bounded in front by the second visceral, or 

 the hyoid, arch (zb). The latter gradually develops a small process 

 backward, which covers over the cervical sinus and has been justly 

 compared by RATHKE with the operculum of Fishes and Amphibia. 

 The opercular process at last fuses with the lateral wall of the body. 

 Thereby the sinus cervicalis, which corresponds to the cavity beneath 

 the operculum which in Fishes and Amphibia covers in the real gill- 

 arches ', is closed up. 



One easily gets an accurate conception of these important processes 



19 



Fig. 156. Frontal reconstruction of the oro-pharyngeal 

 cavity of a human embryo (Rgot His) 11 -5 mm. long, 

 neck measurement. From H.s, " Menschliche Em- 

 bryonen." Magnified 12 diameters. 



The upper jaw is seen in perspective, the lower jaw in 

 section. The last visceral arches are no longer 

 visible externally, since they have moved into the 

 depths of the cervical sinus. 



