DEVELOPMENT OF THE EXTERNAL FORM OF THE IMAGO. 



371 



developed as entirely new rudiments. "We consequently find, in 

 the last larval stage that precedes the pupal stage, correspondingly 

 arranged imagined discs. Each thoracic segment shows four such 

 discs, two ventral and two dorsal (Fig. 183, ba and fa). The ventral 

 discs (ha) become the limb-rudiments. Of the dorsal pairs of discs 

 (fa), that occurring in the meso-thorax becomes changed into wings, 

 that in the meta-thorax into the halteres, while the corresponding 

 rudiment in the pro-thorax yields, in Corethra, the stigma-bearing 

 dorsal process and, in Simidia, a tuft of tracheal gills. If we examine 

 such a rudiment (imaginal disc) of a limb more closely, we see that 

 the limb itself as elsewhere (e.g., in the 

 Hemimetabola) arises as an outgrowth of 

 the surface of the body. The only dis- 

 tinction here found is that the limb- 

 rudiment, as a whole, appears sunk below 

 the level of the surface of the body. It 

 arises at the base of an invagination, in 

 the same way as the head- and trunk-discs 

 in the Pilidium larva of the Nemertini 

 (Vol. i , p. 221), and the rudiment of the 

 lower surface of the Echinoid body 

 in the Pluteus (Vol. i., p. 439). Such 

 instances of the occurrence of rudiments 

 of important parts of the adult body in 

 an invaginated condition might easily be 

 multiplied. For instance, the body-wall 

 of the primary zooecium of the ecto- 

 proctous Bryozoa is found invaginated in 



the larva (as the sucking-disc and mantle-cavity). The lumen of 

 the invagination in which the limbs of Corethra (and of other 

 Holometabola) appear as rudiments was called by Van Eees the 

 periportal cavity, and the sheath which bounds it externally, and 

 which naturally is continuous with the hypodermis, was named the 

 peripodal membrane. 



We must assume that, from the very first, an ectodermal and a mesodermal 

 portion derived from the corresponding layers of the germ-band, take part in 

 the rudiments of the limbs. The ectoderm of these rudiments is in continuous 

 connection with the peripodal membrane, and through it with the hypodermis 

 covering the larval body. WEISMANN was inclined to derive the organs that 

 develop within the limb-rudiments (tracheae, muscles, etc.) from growths of the 

 neurilemma of a nerve joining the imaginal disc. For nerves and tracheal 

 ramifications appear early on the inner surface of the imaginal discs. 



Fig. 1S3. — Diagrammatic trans- 

 verse section through a thoracic 

 segment of a larva of Corethra 

 (from Lang's Text-book), ba, 

 limb-rudiment; fa, wing-rudi- 

 ment ; be and ft, peripodal 

 depressions ; Ihy, larval hypo- 

 dermis ; Ih, chitinous cuticle 

 of the larva. 



