The (ievelopment of the uiinonenital organs of the lamproy. '^Q 



shown in tubules 2, 3 and 4. The nephrocœle contracts suddenly 

 and then extends into the apex of the tubule as a very slender lumen. 

 The blind end of each tubule turns back and exhibits a tendency 

 to fuse with the blind end of the next posterior tubule. This is parti- 

 cularly noticeable in the third (pron^) which even in this stage is the 

 longest in the whole pronephros. Posteriorly the organ in continued 

 into the pronephric duct (d) which has a well-defined lumen. 



It will be observed that the six tubules extend over a space oc- 

 cupied by five and one half myotomes. That these tubules are at 

 first strictly metanieric is shown in a frontal section (Fig. 26) where 

 six tubules are closely applied to the myotomes of their respective 

 segments. The first tubule (pron^) is directed backwards, the last 

 {prou^') forwards. This condition may be explained in the following 

 way. A glance at Fig. 27 shows that the pronephros is wedged into 

 a space between the myotomes on the dorsal and the yolk-laden ento- 

 derm on the ventral side. Since the head and dorsal surface grow 

 forward more rapidly than the underlying portions, the tubules are 

 necessarily crowded together. This process is already begun in 

 stage 3 and continues till the whole pronephros extends over a space 

 of only three segments (conf. Fig. 41) or even less. 



Fig. 30, PI. 3, represents a sagittal section through the pronephros 

 in a somewhat more advanced stage than the one just considered. 

 Here only five tubules are seen occupying the space of three and 

 one half myotomes. The tubules vary considerably in length, the 

 most anterior one being the shortest, the third the longest. The 

 former lies at the anterior end of the collecting duct which unites 

 all the tubules and is continued back into the pronephric duct (d). 

 The segment in which the first uephrostome lies is the seventh behind 

 the otocyst. In its original metameric condition the organ extends 

 from the seventh to the eleventh, or, in case there are six tubules, 

 to the twelfth segment. As the posterior edge of the otocyst is 

 nearly at the same level as the posterior edge of the second gill-cleft 

 (see V. KuPFFER, 1890, p. 524) and just in front of the rudiment of 

 the vagus ganglion, the number of segments may be counted from 

 that of the vagus ganglion. In the stage under consideration the 

 tubules are directed downwards and open by means of distinct nephro- 

 stomes into the unsegmented body cavity just behind the branchial 

 region. This unsegmented cavity arises by fusion of the nejjhro- 

 cœles with one another and with the cavity which has arisen between 

 the parietal layers of mesoderm. Karyokinetic figures in the walls of 



2* 



