DEVELOPMENT OF THE DIGESTIVE CANAL 569 



are turned in, forming an inverted layer constituting the germs of a 

 third layer interpolated between the two chief layers of the eye. 

 (Korschelt and Heider, from Patten.) Patten concludes that the 

 structure of the retina in the larval ocelli of insects is much like 

 that of myriopods, and that the whole eye is constructed on the 

 same plan as that of Peripatus and most molluscs. 



Intestinal canal and glands. The intestinal or digestive canal is 

 primitively divided, as already stated on p. 299, into three sections, 

 of which the anterior and posterior are called respectively the 

 stomodaeum and proctodaeum, and are invaginations of the ecto- 

 derm, forming sacs whose blind ends face the future site of the 

 mid-intestine. The fore-intestine (stomodgeum) in most cases arises 

 earlier than the proctodeeum. Its muscles are derived from the 

 mesoderm. From the stomodoeum arises at an early date an un- 

 paired dorsal invagination out of which develops the ganglion 

 frontale and the pharyngeal nerve. 



The absorption of the ends of the blind sacs of the fore and hind intestine, 

 and opening up of the passage into the mid-intestine, occur rather early in em- 

 bryonic life. In the wasps and bees, as well as the larva of the ant-lion, the 

 mid-intestine remains closed at the end, not communicating with the procto- 

 daeum, which has an exclusively excretory function (Fig. 497). 



The mid-intestine arises from two originally separate rudiments, 

 i.e. the fore and hind endodermal rudiments, which at the outset 

 stand in the most intimate relation with the invagination of the 

 fore and hind intestine. Originating as a simple collection of cells, 

 so closely adjoining these invaginations that Voeltzkow, Patten, and 

 Graber derived them directly through outgrowths of them, they 

 become extended by advancing cell-multiplication until they assume 

 a U-shaped form. The legs of the U-shaped rudiment are in the 

 anterior endodermal mass, directed backwards; those in the poste- 

 rior mass, on the other hand, are directed anteriorly. These legs 

 grow towards each other until they become fused together, forming 

 two paired endodermal streaks, which pass under the primitive band 

 along its whole length, and are fused with it at the fore and hind 

 ends. In these places they stand in intimate union with the proc- 

 todeal and stomodeal invaginations. 



The paired endodermal streaks belong to the lateral portions of 

 the primitive band. As a rule, they lie directly under the row of 

 coelom-sacs (Fig. 539, F). The dorsal wall of the primitive seg- 

 ments stands consequently in intimate contact with the endodermal 

 streaks. On this wall of the primitive segments an active cell- 

 growth takes place, and the cell-material produced in this way, 



