38 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.98 



groove (pi. 8, fig. 61 ; pi. 2, fig. 17). There is no proliferation of cells 

 from the blastoderm by mitotic division such as Wheeler (1889) 

 describes for Blatta. Instead, there is merely a sinking of the cells 

 which are located at the midline. In certain cases this appears to be 

 an irregular process similar to that which has been described for the 

 formation of the mesenteron rudiments. Especially is this true in 

 the region adjacent to the anterior rudiment where the transition from 

 the cells of the lower layer to those of the rudiment is difficult to 

 make out. Posteriorly, the migration of cells appears to become more 

 regular. At the beginning of the second third of the ventral plate 

 there is a gradual transition from this first method of lower layer 

 formation to the second type which has been described above, viz, 

 that in which the migration of cells is combined with the formation 

 of a groove (pi. 8, figs. 62, 63, 64). This method is followed through- 

 out the major portion of the germ band, and the groove is shallow in 

 all places. The cells which form the sides of the groove are destined 

 to form the lower layer. These cells separate from those which are 

 to form the ectoderm, the breaks occurring at the points where the 

 blastoderm turns in to form the walls of the groove. In the meantime, 

 the sides of the groove approximate, but never to form a tube. In 

 some cases the mesoderm-forming cells migrate to the lower layer 

 level in two parallel columns. At a point somewhat ventral to the 

 posterior pole of the egg, the groove deepens abruptly. From this 

 region on to a position only slightly anterior to the posterior mesen- 

 teron rudiment, the sides of the groove widen out below the surface 

 layer, after which the lips of the groove approach one another and 

 fuse to form a true tube with a distinct lumen (pi. 2, figs. 18, 19; 

 pi. 8, fig. 65). In the relatively short region at the posterior end of 

 the embryo, between the posterior mesenteron rudiment and the section 

 of the germ band where a tube is formed, a simple groove is again 

 produced, the sides of which appose as in the middle embryonic region. 

 Along the short length of germ band which forms the posterior 

 extremity of the embryo and which is carried around the terminus of 

 the invaginating proctodaeum, the method of lower layer formation 

 is identical to that which occurs in the anterior third of the egg (pi. 7, 

 fig- 58). 



It has been stated that in the formation of the lower layer of flea 

 embryos, proliferation of cells from the blastoderm does not play a 

 part. Instead, the sunken cells arrange themselves so as to form a 

 relatively simple row the full width of the germ band below the ecto- 

 derm (pi. 9, figs. 69, 70). The details of this process vary according 



