356 Ralph S. Lillie 



The third diapliragm does not appear until a somewhat later 

 stage — typically at the time of formation of the sixteenth or 

 seventeeuth somite. Its uianuer of formation is in ali respeets siniilar 

 to that of the second diaphragm. Its ventral iusertion eoincides 

 with that of the fourth primitive septum; and the latter, as in the 

 case of the third septum, mav persist in its originai position for 

 some time after the diaphragm has become complete. 



At tliis stage (17 somites), the oesophageal iusertion of the third 

 diaphragm is immediately anterior to the stomach. The Oesophagus 

 has tlius evidently undergoue an elongation of about the length of 

 one somite since the time of formation of the second diaphragm. 

 In later stages a section of considerable length intervenes between 

 the oesophageal Insertion of the third diaphragm and the anterior 

 l)Oundary of the stomach. Evidently therefore there is in progress 

 a backward Prolongation of the Oesophagus. The mannet in which 

 this Prolongation takes place is peculiar: it takes place in part 

 probably by a process of simple backward growth; but it seeras to 

 he chiefly due to an extensive histolytic transformation of the 

 anterior region of the primitive stomach. The nature of this 

 transformation may be determined in part from the examination of 

 longitudinal sections of larvae of slightly later stages. In a larva 

 of 23 somites, both diaphragms are completely formed; and in the 

 fifth somite, a short distance behind the third diaphragm, the chauges 

 alluded to are api)arently in active progress. The Oesophagus is 

 bere almost occluded by what appears to be a strong constricting 

 niusclc of deeply staining circular fibres, situated at the point of 

 junction of Oesophagus and stomach. The anterior stomach-region 

 seems to be undergoing a process of dissolution preparatory to its 

 transformation into oesophageal tissue. The typical unaltered stomach 

 cells are lightly staining, and homogeneous in appearance; they 

 are non-ciliated — except in a groove along the median ventral 

 wall (Piate 23, Figs. 16, 17, 30) — and contain a few deeply staining 

 granules. As the region of constriction is neared, however, their 

 appearance undergoes a mavked alteration; they become greatly 

 narrowed and elongated, and drawn out at their inner euds ajìparently 

 into long tapering flagella. Stili farther forward the cells are 

 vacnolated and largely l)roken down. On passing stili farthor for- 

 ward the vacuolation becomes less marked; and finally the vacuo- 

 lated cells pass by a graduai transition into the ordinarj cubical 

 ciliated cells of the Oesophagus. 



