130 M\!\l'. \(;KU-|-|,TIK \l, i:\ I'I'KI M I'NT STATION'. T()0,^. 



S(.'i;nu'nl. Thu^t' ul ilu' lour posU'i-ior alxloinin.-it sc^iiu'iils arc 

 \'(.'ry small and do iml coiiliiuu' to (K'wlop. 



In iIk' laid" liU' ol llu' (.'luhiA 1 1 iIktc is a marked concentration 

 of tlic ventral nervous system. The i^-anmiia in llic abdominal 

 reg;ion arc reduced to a nerve cord and the g'anglionic part of the 

 chain is confined to the thorax. When the lower layer was 

 formed, iwn mas.ses of cells, one anterior and the other posterior, 

 developed in a manner distinct from that of the m'reat mass of 

 the lower la_\"er. These two cell masses become the rudiments of 

 the develo]Mn,Q- enteron. As the ijrowth of the embryo j^oes on 

 the\' s])read farther and farther t<)war(l the center of the body 

 forming- two u-shaped bands of tissue. Finally the two bands 

 come toq-ether and fuse on the ventral side, the dorsal side 

 remaining- open until after rotation takes place. While the 

 changes leading to the establishment of these sets of organs are 

 going on, the germ band is increasing rapidly in size, both longi- 

 tudinally and laterally. The posterior end of the embryo 

 becomes strongly flexed, so that it extends toward the posterior 

 end of the egg and lies along the v-entral side of the abdomen. 

 The appendages have increased in length and the mandibles and 

 maxillae have become lobed. 



On the fourth or fifth day after the egg is laid, rota- 

 tion of the embryo takes place. The amnion and serosa fuse 

 at the point where they previously separated. The fused 

 envelopes are ruptured and the embryo begins to be drawn out 

 through the opening in the membranes. The head of the 

 embryo passes anteriorly along the ventral surface of the egg 

 until it comes to occupy the anterior end of the egg, the posterior 

 part of the embryo taking up its position in the posterior end. 

 By this rotation the embryo has come to a position so that its 

 ventral surface is next to the ventral surface of the egg. The 

 embryonic envelopes, during the process of rotation, are carried 

 around so that they lie as a shrunken mass on the dorsal side of 

 the embryo, making a provisional body wall. The serosal part 

 of the envelope soon becomes a thick mass of cells, with prom- 

 inent nuclei, situated just posterior to the head, making the 

 dorsal organ. The yolk mass of the egg, which by this time has 

 been much diminished in bulk, is brought so that it lies inside of 

 the body in the region of the growing enteron. The dorsal 

 walls of the enteron now grow rapidly and soon inclose 



