I 1 8 WHEELER. [Vol. VIII. 



while those of the tenth have grown smaller and approached 

 the median ventral line. They have, in fact, grown too small 

 to contain the ampullae which are drawn away from them and 

 lie between the ninth and tenth segments a little in front of 

 the abortive appendages. At the same time the inner faces of 

 the ampullae have become flattened and applied to each other 

 in the median line. Each of these sacs has a pointed tip 

 directed caudad. The more arcuate course described by the 

 ducts in this stage is undoubtedly due to the mutual approxi- 

 mation of their terminal ampullae. The appendages of the 

 eleventh segment, the cerci {cc. [<?/"]), which in the pre- 

 ceding stage were rounded like the other abdominal append- 

 ages, have become oval. 



A more advanced embryo is represented in Fig. 44 (some- 

 what younger than Stage K). The appendages of the tenth 

 segment {ap^'^) have almost completely disappeared. Those 

 of the ninth segment, the future styli (^/9) have lengthened 

 and now point outwards and forwards. The cerci have grown 

 more pointed. The terminal ampullae lie completely in the ninth 

 segment, having shifted their position headwards. The move- 

 ment takes place in such a way that what were the posterior 

 faces of the ampullae in the younger stage (Fig. 43) are applied 

 to each other, while the pointed tips are directed forwards. 

 The ducts are thereby rendered still more arcuate towards 

 their terminations. An intermediate stage in this singular 

 movement is shown in Fig. 45, where only small portions of 

 the posterior faces of the ampullae are as yet applied to each 

 other. Comparison of Figs. 43, 44 and 45 shows that the 

 pointed tips remain united and move forward while the sur- 

 faces of mutual contact are being shifted. Finally in Fig. 46, 

 which represents the abdominal end of an embryo ready to 

 hatch, we see that the terminal ampullae have increased con- 

 siderably in size at the expense of the thickness of their walls. 

 They have also lengthened, and brought still more of their 

 surfaces in contact in the median line. The pointed tips of 

 the ampullae extend into the eighth segment. It may also be 

 noted that the points where the spcrmaducts meet the ampullae 

 have moved forward. The appendages of the tenth segment 



