236 EMBRYOLOGY [CH. 



When the primitive groove was formed, its anterior end 

 deepened into a pit, the stomodaeal invagination, marking the 

 position of the future mouth of the embryo. This pit gradually 

 deepens and lengthens out posteriorly, so that its blind end at 

 last comes to project close up against the yolk-cells in the region 

 of the future mid-gut. At a later period, when the anal plate 

 (see below) of the embryo is formed, a similar but smaller invagina- 

 tion appears at the extreme posterior end of the embryo, on the 

 eleventh (or possibly the twelfth) segment. This pit is the procto- 

 daeal invagination, marking the position of the future anus. 

 It grows longer in an anterior direction, and thus finally pushes 

 its blind end close up to the yolk-cells in the region of the mid-gut 

 in front of it. 



When these two pits are small, a cluster of small but very 

 distinct cells appears around the blind end of each, beneath the 

 mesoderm. These clusters soon become crescent-shaped, and then 

 spread out like two pairs of horns advancing towards one another 

 on either side of the mid-gut. Finally the two horns of each 

 side meet and coalesce. These form the endoderm cells (fig. Ill, 

 en], arranged, as already seen, in a series of nests close outside 

 the vitellophags. But only the middle portion (w 2 ) of the mid-gut 

 is thus enclosed. According to Tschuproff, a short anterior 

 part (MJ) is formed from the end of the stomodaeum, by constric- 

 tion off, where its blind end opens to the mid-gut ; while a similar 

 short posterior part (w 3 ) is formed from the proctodaeum. 



We thus have this remarkable result in the Odonata, that the 

 whole of the alimentary canal, except the middle portion of the 

 mid-gut, is of ectodermal origin. 



Now it is this middle portion which is still in dispute. The 

 two views may be stated thus : 



(a) Grassi (1884), Nussbaum, Kowalevski, Wheeler and 

 Hirschler, with others, consider that the "nests" of cells which 

 form the mid-gut are undoubtedly derived from the lower layer. 

 Thus they are correctly termed endoderm cells, and the middle 

 portion of the mid-gut is of endodermal origin. 



(6) Heymons (1895), with some more recent supporters, holds 

 that these "nests" are of ectodermal origin, since they first appear 

 close up to the stomodaeum and proctodaeum, which are admittedly 



