MISMOIKS OF THE NATIONAL ACADEMY OF SCIENCES. 



415 



Tlit^ two <livi8ioii8 of tbe foregut, iesoi)hagus, ami masticatory stomach, have the relations 

 alrea4ly (lescribed. The wall consists of a single layer of tall columnar cells. In Iht^ masticatory 

 division tlie wall lias hegun to thicken next the yolk screen, its cells being cuneiform, and the 

 nuclei elongated and crowded below the surface. From the anterior wall, muscle or couuective 

 tissue cells extend forward under the braiu. 



niyi in 



Fic 1. — DiayriiiiiH iit" transverHO SfCtiona tbroagh the aliiiiontary irai-t of an enibrj'o of Alphcus tiaulci/i M-hiuli in 

 nearly re;w!y Ut liatrli. to allow the origin of the ;j;a8lric ^land from the itostero hitei'ul lobes of tbe niiflt;iit. Si-ction 

 I tuts thi' hiiiiljiiilaiiil lobta of the "liver," Section III tbe biml^ut wbeio it mrrues into the lueaeutelou. gij', gg', 

 StrcoiHlaiy lobulertof iiig ' ; 110, hiiid<;nt ; mj', poatero-lateral lobea of loid^tit. 



The development of the meseuteron can be understood by reference to Figs. 102-1G5, IGtS, and 

 185. The endoilermal e|)ithelium spreads by the division of its own cells and by accession of cells 

 from the yt>lk, both forward over the nervous system and upward against the sides of the body. 

 This is shown in the series of horizontal sections (Fig. l(i2-lC5). Fig. UJS which is from au 

 embryo a little more advanced, shows that the endoderm is rising from the nervous cord near its 

 point of llexiiie, into a transverse vertical fold. Simultaneously with the upward growth of this 

 ventral foltl, two dorsal longitudinal folds grow downward, aiid-linally unite with the ventral fold 

 antl with each other, thus coiistrictiug off from the alimentary tract two lateral pouches, the pri- 

 iL-ary lobes of the " liver." The folils grow forward and the constriction i>roceeds gradually with 

 the growth of the embryo. This process is illustrated by the diagrams (Fig. 1) which were drawn 

 from an embryo near the point of hatching. The histology of the endoderm as shown in Fig. 173, 

 is es.seiitially the same as in the previous stag^'. The cells are inismatic, and tlie nucleus .spherical, 

 and, as in all stages, tilled with numerous nucleoli or chromatin balls. The cell walls are very 

 delicate and the protoplasm often contains large vacuoles. 



9S> 



Fic;.'.>. — Seiiii<IiaKraniniatic representation of the alimentary i met and iin ;i|i|ien<la):ea in the 

 lirat larva of Atiihrim taiilnii. The niidille lin^' of Ibe body i» also shown. F.S, foreRut . gg 1-3, 

 secondary I obiili« of posliio lateral lobe of luidgnt; //S, hindgut; iiiy. uiidyul ; ing 1-3, an- 

 terior, lateral, and post«ro-lateral divisions of luidgut; nio, mouth. 



