THE ALIMENTARY CANAL 87 



mass. From here these cells migrate outward, arranging themselves in a 

 layer outside the nutritive mass and thus forming the mid-gut epithelium. 

 That which remains of the bipolar rudiments later disintegrates after a 

 curious history as described in Part II. 



In the head louse (Pediculus) and in the biting lice (Lipeurus haculus, 

 Gyropus ovalis) the mid-gut epithelium develops from bipolar mesenteron 

 rudiments. This is the case also in the bugs Pyrrhocoris apterus and 

 Rhodnius prolixus. In the summer eggs of the aphid genera Rhopalo- 

 siphum and Aphis, Hirschler (1912) found that anterior and posterior 

 mesenteron rudiments gave rise to a small entodermal strand of few cells 

 without lumen which formed between the ends of the stomodaeum and 

 proctodaeum but later are crowded out by these invaginations. Thus 

 the entoderm is restricted to form a suspensorium, the definitive mid-gut 

 therefore being w^holly ectoderm. The yolk cells take no part in the 

 formation of the mid-gut. The anterior and posterior rudiments 

 Hirschler called the "secondary entoderm." No ribbons are formed in 

 this process. This type of development resembles that found in the 

 isopod crustaceans Porcellio and Armadillidium as described by Good- 

 rich (1939). 



Shinji (1919) states that in the coccids Pseudococcus mcdanieli, 

 Lecaniodiaspis pruinosa, and Icerya purchasi the mid-gut arises from 

 rudiments consisting of the entodermal cells situated at the posterior end 

 of the embryo, growing in as two parallel layers extending cephalad from 

 the caudal extremity, where the three germ layers arise. The cells then 

 multiply rapidly and form a coiled tube which pushes forw^ard until its 

 anterior end meets the tip of the stomodaeum. 



NEUROPTERA AND TRICHOPTERA 



The mesenteron rudiments in the alder fly {Sialis lutaria), according 

 to Strindberg (1915), develop at the extremities of the inner layer. From 

 each rudiment two sheets of entodermal cells arise, one on each side, which 

 grow in length and width to enclose the yolk thus forming the mid-gut 

 epithelium. 



Tichomirowa (1890) states that in Chrysopa, another member of the 

 order Neuroptera, yolk cells give rise to the mid-gut epitheHum, This 

 work requires confirmation. 



Patten (1884) has described a somewhat similar type of development 

 for the caddis fly (Neophylax concinnus). In this species cells do not 

 remain in the yolk while the blastoderm is forming, but later some 

 migrate back from the blastoderm during the formation of the embryonic 

 envelopes. These amoeboid cells with large nuclei, which are gradually 

 distributed through all parts of the yolk, were called by Patten "yolk," 

 or "entoderm," cells. They arise from any point in the blastoderm by 



