THE ENTODERMAL TRACT. 165 



gram, Fig. 181, except along its lower border, represent groups or pairs of Mall's 

 lobules. Professor Stohr notes that "a division into lobules in the interior of 

 the spleen is impossible." The arrangement of lobules at the periphery suggests 

 an ill-defined cortex. Lobes have also been described, corresponding with the 

 main branches of the splenic artery, but the lobes are not generally recognized. 

 The spleen may present inconstant subdivisions, which sometimes produce 

 detached portions called accessory spleens. 



THE ENTODERMAL TRACT. 

 DEVELOPMENT OF THE MOUTH AND PHARYNX. 



In a previous section the early development of the pharyngeal pocket 

 of entoderm has been described and illustrated (Fig. 20). This 'pharynx' 

 of the young embryo is to produce the fore part of the intestinal tract in- 

 cluding the pharynx, oesophagus, and stomach of the adult. Its anterior 

 extremity encounters the ectoderm at the bottom of a depression. The 

 ectoderm and the entoderm there fuse to make the oral plate (Fig. 185), 

 which becomes thin, ruptures, and disappears. Just anterior to the plate, 

 in the median line, the ectoderm sends a gland- 

 like projection toward the brain. It branches 

 and becomes detached from the oral ectoderm, 

 lying in the sella turcica of the adult. It is 

 known as the anterior lobe of the hypophysis, and 

 it will be described with the brain, from which 

 the posterior lobe develops. The ectoderm in 

 front of the oral plate forms also the epithelium 

 of the lips and of the peripheral part of the mouth 

 including the enamel organs, as has already been 

 described. No line of separation between the 

 ectoderm and entoderm can be found in the adult. 



The entoderm of the mouth and pharynx 

 constitutes the epithelium lining a broad cavity 

 flattened dorso-ventrally. It produces a succession of paired lateral out- 

 pocketings which meet corresponding ectodermal depressions. Ectoderm 

 and entoderm fuse where these meet, making plates similar to the oral 

 plate, and in fishes these rupture to produce the branchial clefts (gill 

 clefts). Their arrangement in a young dog-fish is shown in Fig. 186. 

 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 anterior to them and having a more dorsal aperture. Gill clefts 

 and spiracle occur on both sides of the fish. In mammalian embryos 



FIG. 185. DIAGRAM SHOWING 

 THE RELATIONS BETWEEN 

 ECTODERM AND ENTO- 

 DERM IN THE MOUTH OF A 

 MAMMALIAN EMBRYO. 



a. 1., and p. L, Anterior and 

 posterior lobes of the 

 hypophysis; m. t., medul- 

 lary tube; ph., pharynx; 

 o. p., oral plate; x. and y., 

 ectoderm which produces 

 the lip and teeth of the 

 lower and the upper jaw 

 respectively. 



