298 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



still exists in the yolk, but by this time the chromatin material has 

 disintegrated almost entirely (Fig. 239). The cleavage nuclei in the area 

 nearest this last remnant of the polar body are not so numerous as in other 

 regions of the periphery, and no cell walls have begun to form, although 

 they are well along in development on the opposite side of the egg. This 

 would indicate that as long as the polar body lasts, its presence prevents 

 the cleavage nuclei from entering the periphery. 



The oosome is an irregular mass of granular material located at the 



posterior pole of the egg. When 

 stained with either iron haemo- 

 toxylin or Delafield's haemotoxylin 

 it appears much darker than the 



nb / ^ '' ''■ '>" '' ^>- '^^?o to"-"* ''W^% ""^^^ ^^ ^^^ cytoplasm of the egg. 



^^ * '^■^''■'' "" ■'-^m.-^ i^ j^^y form a saucer-shaped disk 



lying between the yolk and the 

 cytoplasm (Fig. 234) or a smaller 

 but very thick mass extending for 

 some distance into the yolk (Fig. 

 235). In either case it is not 

 homogeneous but contains large 

 globules or irregular bodies of mate- 

 rial which stain much lighter and 

 appear to be identical in composi- 

 tion with the peripheral cytoplasm. 

 Yolk globules also are scattered 

 through the germinal cytoplasm 

 but not to the same extent as the 



Fig 2iO-Brachyrh^nus Longitudinal Ughter staining material, 

 section of blastoderm {bid) . {gc) Germ cells. As they approach the posterior 



(pb) Polar body. p^j^^ ^^^^ ^f ^^le cleavage nuclei 



come into contact with the germinal cytoplasm (oosome) which breaks up 

 and gathers around them in large masses. They move on to the periphery 

 but do not leave the walls of the forming blastoderm (Fig. 239). They 

 are the germ cells, and their development from this point is entirely 

 distinct from the other cleavage nuclei. The blastoderm cells, as they 

 increase in number and acquire walls first on the outside, then on their 

 lateral faces, become columnar in form except where they are interrupted 

 by the germ cells and where they are retarded by the disintegrating polar 

 body. Before the inner walls of the blastoderm have formed, the germ 

 cells move inward to form a syncytium between the yolk and the blasto- 

 derm cells (Fig. 239). When the cleavage nuclei move out from the 

 center of the egg, several of their number are left behind to become yolk 

 cells. Others, as they reach the germinal cytoplasm, seem to gather up 



