52 



dorsal part of the blastoderm toward the posterior end, so that the 

 anterior end of the blastoderm is now completely enclosed by the 

 serosa. This retreating of the attachment of the amnion apparently 

 takes place by a kind of progressive delamination from the blasto- 

 derm cells successively farther back. The anterior end of the germ 

 band has retreated somewhat and the anterior cell-mass has in- 

 creased in size. The middle groove and the middle plate have grown 

 farther backward but have not reached the posterior end of the germ 

 band, while at the anterior end the surface is again even, the middle 

 groove having grown over. The transverse depression at the pos- 

 terior end of the germ band has become somewhat deeper, but the 

 blastoderm cells at the posterior edge of this depression have grown 

 forward but little if any farther than in the last stage described. 



In a stage five days old (PI. IV, Fig. 15), the serosa has grown 

 backward on all sides almost to the posterior end of the blastoderm, 

 enclosing the large cells of the blastoderm and the germ band. At 

 the posterior end of the germ band the serosa did not unite with the 

 forward-projecting cells from the posterior end of the transverse 

 groove, but continued to grow on backward, enclosing those cells 

 with the posterior end of the germ band. 



At the position of the transverse groove mentioned in preceding 

 stages, the germ band dips downward and backward diagonally and 

 then continues to grow toward the posterior end, following the thin 

 layer of peripheral protoplasm. At this stage the backward growth 

 from the lower end of the incline has proceeded only for the length 

 of a few cells. From that point backward toward the posterior end 

 of the blastoderm the layer of inner protoplasm, which although very 

 thin is easily distinguishable, rises again toward the surface, leaving 

 a very broad depression, in which lie a mass of large blastoderm cells 

 covered by the serosa. 



At the anterior end the germ band turns upward and then back- 

 ward again on the dorsal side, and since the dorsal and ventral parts 

 are now connected by a layer of cells, this gives to the anterior end 

 of the germ band the appearance of a closed tube with the lumen 

 opening backwards. The layer of thin epithelial cells forming the 

 dorsal surface of this tube extends backward at this stage, as a deli- 

 cate layer, to about the level of the transverse groove near the pos- 

 terior end of the germ band on the ventral side. From this point 

 the wall of the tube, which widens somewhat here, extends backward 

 as the inner protoplasmic layer to the posterior end of the blastoderm, 

 where, with the same layer from the sides and the ventral surface, it 

 forms the other closed end of the tube. Betw^een the antero-dorsal 



