61 



together of the anterior and posterior ends of the embryo and by 

 the dorsal growth of the sides of the germ band. 



The germ band has widened considerably, and now covers most 

 of the ventral surface of the yolk. Figure 28 represents a view of 

 the anterior end of the germ band at this stage, showing the pro- 

 cephalic lobes in front, the beginning of the evaginations represent- 

 ing the mandibles, maxillre, and labium. At this stage there are 

 only the faintest traces of the three pairs of thoracic appendages, 

 and the invagination of the proctodseum has not yet begun. 



In the stage/epresented by Figure 29, the curvature of the embryo 

 is still greater, the anterior and posterior ends more closely approxi- 

 mating each other. The dorsal thickening between the anterior and 

 posterior ends is present as before, and as a rule is somewhat larger 

 than in the preceding stage. The segments are much more clearly 

 differentiated, there being ten abdominal segments. The procephalic 

 lobes have increased in size, and the lateral edges of the germ band 

 have grown farther dorsally. The labrum is shown in Figure 30, 

 which represents a dorso-frontal view of the same stage. Figure 

 30 shows also the extent of the invagination of the stomodseum and 

 the lobe-like appendages representing the three pairs of mouth parts 

 and the first two pairs of legs. Figure 31, which represents a slightly 

 different view of the same stage, shows also the last two pairs of 

 thoracic appendages and the first pair of abdominal appendages, 

 and on these segments also appear the first three pairs of spiracles. 

 The invagination of the proctodneum is shown at the posterior end. 



Figure 32, Plate VIII, represents a later stage, in which the 

 labrum is well developed, pushing out in front of the invagination 

 of the stomodseum, which has pushed farther inward. In fact, the 

 posterior border of the labrum is continued inward to form the an- 

 terior wall of the invagination. The procephalic lobes are larger and 

 the germ band has grown farther over the yolk. The proctodreum 

 now appears as a distinct U-shaped thick-walled invagination. The 

 dorsal thickening, made up of a cluster of cells, has disappeared in 

 this stage. In stages just a little earlier than this, a constriction de- 

 velops at the base of this thickening. This fact, together with the 

 fact that in such stages the thickening is very easily broken away 

 from its attachment as the embryo is moved about in the clearing 

 agent, leads me to believe that it takes no part in the development of 

 the embryo. The membrane covering the yolk, from which this 

 cluster of cells is formed, is homologous with what we have called 

 the amnion in Cauiponotns. This clu.ster of cells, then, seems to 

 correspond with the so-called amniotic dorsal organ which Wheeler 



