372 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



This migration has been noted by Voeltzkow (1889), Metsehnikoff 

 (1866), Will (1888), and Lassman (1937), though Graber controverted it. 

 In other respects Graber's account is correct as regards the migration of 

 other lateral blastoderm cells into the interior. It seems probable that 

 some of the yolk cells come from cleavage nuclei that had remained 

 behind, as Blochmann, Graber, and Kowalewsky have already noted, 

 but the majority of yolk cells arise from the return migration of blasto- 

 derm cells, though only after the cells of the periphery are closely crowded. 

 Those which remain behind do not do so a -priori but turn back when they 

 have nearly reached the surface. In Calliphora only one to three such 

 cells come from the cleavage cells that had remained behind, but in 

 Lucilia they are more numerous. This relation is therefore variable in 

 the Muscoidea. The usual view of the function of the yolk cells is that 

 they are concerned in the breaking down of the yolk substance to render 

 it available for the blastoderm. Noack (1901), however, believed that 

 these cells, in the muscids at least, by their pseudopod-like processes are 

 for the mechanical purpose of yolk support, perhaps in addition to their 

 other use. 



The cleavage nuclei reach the surface simultaneously, but the germ 

 cells remain separated from the periphery by yolk. Later a second layer 

 of cytoplasm (pr. i) is said to be formed under the blastoderm and is 

 separated from it by a thin layer of yolk substance. Noack, however, 

 states that this is only in part true. The distribution is not irregular, as 

 Graber has stated, but is as shown in the accompanying jfigure (Fig. 

 327Z)). Dorsally it is very thin, but it is thicker ventrally and not 

 developed posteriorly. 



Absorption of nutriment by the blastoderm does not begin until the 

 blastoderm cells extend (in depth) in contact with the yolk. The yolk 

 elements absorbed by the cells do not appear to differ from the other yolk 

 elements. After a time the lower blastoderm cell wall is formed so that 

 the yolk no longer is in direct contact with the protoplasm of the cell. 



It should be stated that the cells of the developing embryo are 

 columnar, whereas the undifferentiated cells of the original blastoderm on 

 the dorsal side are quite flat. Since the latter, though they change but 

 little in character, function as an envelope, they have been designated as 

 the serosa by some writers, Graber among others. The differentiation of 

 the germ band consists rather of a thinning of the dorsal and lateral 

 sides of the blastoderm than of a thickening on the ventral side. 



A surface examination of the developing egg shows that early in the 

 formation of the embryo two transverse furrows develop which extend 

 around the egg and divide it into three divisions, the middle section 

 between the furrows on the ventral side where the middle plate later 

 forms, being again divided by five lines into six divisions of about equal 



