44 T. H. MORGAN. 



pouch to the base of the rudimentary third pair. This pair of 

 appendages is filled with embryonic mesoblast, and the ectoblast 

 covering the appendage is thrown into a series of crenations 

 beneath the cuticle, so that it would be much longer if extended, 

 and is in condition to expand rapidly at the moulting period. 

 This folding of the walls of the undeveloped appendage was 

 seen also in all of the pairs of walking legs at the stage pre- 

 ceding their extension, and makes intelligible the great increase 

 in length of the appendage when the next moult takes place. At 

 the point of union of the gut-diverticulse of the seventh appendage 

 to the central digestive tube there is formed in the angle a small 

 posterior pair of ventral ganglia. These were noc seen in sur- 

 face views, but lie above and behind the last pair of ganglia seen 

 in the figure (XVIII), and belong to the rudimentary abdomen. 



The ' anlage' of the reproductive organs is divided anteriorly 

 into two parts, lying on each side of the dorsal side of the mid- 

 gut, and run as far forwards as the base of the first pair of 

 walking legs. Behind these divisions are united into a single 

 mass in the region of the second pair of walking legs, and dis- 

 appear at the anterior part of the third pair. There is a well- 

 developed heart lying dorsal to these reproductive cells. 



In the next stage we have a change which causes the larva to 

 approximate much more nearly to the adult structure. The most 

 important part of the change consists in the almost total loss of 

 the first pair of appendages. See Figs. XIX and XX, Plate VI. 

 Looking at the animal from the ventral surface we see no trace 

 of this pair of appendages. Turning the animal over (XX) we 

 find two projections at the anterior dorsal border where the 

 first pair formerly arose. Each projection has two segments. A 

 change has also taken place in the .second pair, for these have 

 now doubled the number of segments, and, except in size, have 

 the adult structure. The third pair of appendages (XIX) has 

 increased greatly in size, and now these form a pair of prominent 

 projections on each side at the base of the proboscis. Each is 

 bent almost at right angles back onto itself, and lies closely 

 applied to the body in the space between the proboscis and the 

 first pair of walking legs. Externally they do not show divisions 

 into segments. The only point to note in the walking legs is 

 that the fourth pair have now eight segments. The first three 



