No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 21 



each become trilobed, while the metathoracic leg (/S) already 

 exhibits unmistakable traces of its characteristic thickening in 

 the larva and imago. The pleuropodia (//. {ap^)) stand out clearly 

 from the edges of the first abdominal segment. Shining 

 through the stretched ectodermal layer of the abdominal seg- 

 ments may be seen the paired mesodermal somites (coc), or 

 mesomeres. The anal plate with its pair of cerci {cc. (<?/")), 

 and the anus are definitely established. A faint neural furrow 

 runs from the mouth to the anus, and in the thoracic region 

 faint metameric indications of the ganglia are apparent. All 

 these important changes have taken place within the yolk during 

 the transition of the embryo. This renders their study on 

 hardened material very difficult, for although the embryo may 

 be dissected away from the yolk, it is so much curved that it 

 can be mounted only in pieces, and the yolk is at this period 

 so difficult to cut that only fragmentary series of sections can 

 be obtained. 



One of the most interesting changes undergone while the 

 embryo is still in the yolk is the appearance of the labrum. In 

 Fig. 6 (Stage F) the labrum is a distinctly unpaired circular 

 appendage. But that it has a paired origin I infer from a 

 transverse section, part of which is represented in Fig. 35. 

 This passes just in front of the mouth of an embryo but little 

 older than Stage E. The appendage {Ib^ is here seen to be 

 distinctly bilobed although it does not yet project beyond the 

 general level of the head. This bilateral condition is speedily 

 slurred over and the organ grows into an unpaired and in 

 most embryos perfectly circular disk overhanging the mouth. 

 Very rarely, as in Fig. 7 it may show traces of its paired origin 

 even during later stages. 



Let us return to the indusium which we left as a thin 

 round plate gradually spreading over the yolk just beneath the 

 ventral serosa. The outlines of this plate are not always cir- 

 cular but exhibit traces of lobulation (Fig. 5). The spreading 

 is at first uniform along its whole circumference so that the 

 organ soon assumes the shape of a circular scroll clasping the 

 ^^^. Its lateral edges approximate on the dorsal surface just 

 over the ventral face of the embryo but are temporarily arrested 



