MORPHOLOGY OF CYCLOSTOMATA. Ô/Ô 



This horizontal partition is found at three levels. The most 

 dorsal is well developed, the ventral is a mere trace, and the 

 middle is intermediate between the above two. I can not state 

 at present anything definite as regards the significance of this 

 striictare. 



22. The convolution of the pronephric tubule takes place in 

 Stage IV. With tlie growth of the myotome, the collecting 

 duct is prolonged ; consequently the connecting points of 

 the tubules with the duct are farther removed from one another 

 than before, whilst the nephrostomes retain their original 

 position ; so that, the two posterior pairs of the tubules are placed 

 in an oblique direction from dorsal and caudal to ventral and 

 cranial. Each tubule is, then, convoluted in a cranio-caudal 

 direction between the heart and the lateral peritoneal wall. In 

 older stages, the tubules are coiled in all directions, until the 

 chest cavity becomes filled up with the convolution of the tubules. 



23. Up to Stage vr, the nephromeres and the myomeres 

 exactly coincide one above the other in position. This period is 

 ver}'- long in comparison Avith other Craniota. As the develop- 

 ment proceeds further, the pronephic tubules are however shifted 

 gradually backward, so that, in Ammocoetes 10 mm. long, the 

 myotomes are already not situated upon the tubules pertaining 

 to each of them. In later stages, nothing of the relation can be 

 traced. 



24. The segmental duct is looked upon as being brought 

 about by the union of a series of the abortive pronephric tubules 

 in about 12 somites lying posterior to the eighth somite^\ The 

 Anlage is laid in the parietal layer of the nephrotome iu exactly 

 the same manner as in the pronephric tubules of the glandular part. 



1) Iu the 9th somite, the aborted tubule actually forms the duct iu the segmeut. 



