314 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



that part surrounding the embryonic region, may now be designated as 

 the "trophamnion." 



The first cleavage of the embryonic nucleus is completed at about the 

 third day (Fig. 2615), two embryonic nuclei being then visible in the 

 embryonic region. By the fifth day a second cleavage has taken place, 

 producing four nuclei, the parasite having meanwhile considerably 

 increased in size and usually become wholly surrounded by host tissue. 



About the sixth day after oviposition the parasite body becomes more 

 elongate, and each of the two paranuclear masses {nu. p) as well as the two 



Fig. 262. — Platygaster. Section through twinning stage of the parasite body. About 

 six days old. {am. t) Trophamnion. (emb) Parasitic body, {host) Host tissue, {nu. p) 

 Paranuclear mass. 



parasite bodies (emb) divides, thus forming four of each (Fig. 262). Each 

 half of the embryonic region is now a true germ and is structurally the 

 same as a three-day-old parasite body after the first cleavage. No 

 further major division of the embryonic region or later developing blastula 

 stages takes place, but each germ develops directly into the blastula and 

 later embryo stages and finally into a larva. The original egg (or 

 parasite body) therefore develops into twin parasites, structurally inde- 

 pendent of each other, held together solely by the host tissue (host) that 

 surrounds them. The nuclei of each germ undergo repeated divisions 

 until a blastula (emb) is formed which at first is spherical (Fig. 263^), the 

 paranuclear masses (nu. p) of the trophamnion (am. t) having become 



