262 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



When the stage of the blastoderm development just described is 

 reached, the maternal symbiotic organism passes into the mycetom 

 plasma. Contrary to the views of Hirschler (1912), Toth states that he 

 has never observed symbionts in the summer eggs of aphids before blasto- 

 derm formation. As already described, the yolk-cell syncitium gradually 

 grows around the mycetom, becoming thinner and membranous. From 

 the inner surface of the syncitium, partition walls arise which divide the 

 mycetom syncitium into mycetocytes (Fig. 187). The walls begin devel- 

 opment at the anterior pole of the egg but not until the invasion of the 

 microorganisms (symbionts) is complete and the infection pore is closed 



(Fig. 187). 



After the invasion of symbionts into 

 the interior of the blastoderm has 

 begun, the margin around the opening 

 is turned inward. When the mycetom 

 is filled with microorganisms, the egg 

 follicle closes, which prevents more 

 symbionts from entering. Following 

 this the blastoderm also closes. Mean- 

 while the germ band which has formed 

 on one side of the blastoderm backs 

 further into the interior, dragging with 

 it the portion of the blastoderm that is 

 to form the amnion. 



The germ cells become evident when 

 the invagination of the caudal end 

 of the germ band into the interior is 

 well under way and the invasion of symbionts is nearly completed. 

 Rapid cell division takes place until a mass of cells is formed at the tip 

 of the tail (Fig. 187), the mass being pushed passively inward toward the 

 anterior pole. As development proceeds, the caudal end of the germ 

 band is reflected, the band assuming an S-shaped figure (Figs. 18SC,D,E). 

 About this time the beginning of the body segmentation and the 

 invagination of the stomodaeum are in evidence. The proctodaeal 

 invagination occurs later. 



The development of the mesoderm and the formation of the mid- 

 gut have been described by Hirschler (1912) for Rhopalosiphum nymphae 

 and Aphis rosae. In these insects the inner layer develops along the 

 median longitudinal line of the germ band by the overgrowth of the lateral 

 ectoderm bands without the formation of a distinct gastrular furrow. 

 This inner layer forms a single layer of cells. Distinct clumps of cells 

 at each extremity of the inner layer, such as described for the Lepidoptera, 

 are not found in the aphids. 



Fig. 187. — Hyadaphis sj. 

 Germ band, {gc) Germ cells. 

 Mycetom. (ser) Future serosa, 

 Yolk-cell cap. 



(gb) 



imyct) 



iycc) 



