The development of the uriuogenital organs of the lamprey. 3 



Stage 16. Length 15 mm. "Killed in chromic acid." 



III. A lot (stage 17) of specimens of Ammocœtes of Pe^rom^/^on 

 fluviatilis 21 — 26 mm long, obtained from Prof. Vejdovsky of Prag. 

 These had been fixed in Flemming's fluid. 



IV. A specimen of Ammocœtes (stage 18) 47 mm long, fixed 

 in chromic acid. This is also probably a larva of P. marinus dorsatus 

 Wilder, and was taken by Prof. Gage in New York State. 



V. Three specimens of Ammocœtes (presumably of P. fluviatilis) 

 from the collection of the Zoological-zootomical Institute at Würzburg. 

 They were fixed in chromic acid and represented three different stages: 



Stage 19. Length 7 cm. 

 Stage 20. Length 9,5 cm. 

 Stage 21. Length 17 cm. 



VI. One transforming Ammocœtes (P. marinus dorsatus Wilder?) 

 obtained from Dr. H. P. Johnson (stage 22). It was 16 cm long. 

 The label bore the legend: "Susquehanna River. Sept. 19'^» 1893. 

 V4 "/o chromic acid." 



VIL Stage 23, Adult specimens of Petromyzon marinus from 

 Naples and from Massachusetts ^). 



In studying this material the usual embryological methods of 

 cutting and staining sections were employed except in stages 1 — 12. 

 In these it was necessary to overcome two special difficulties, the 

 obscurity caused by the accumulation of yolk granules in the cells 

 and the curvature of the longitudinal axis of the embryo. The former 

 difficulty was overcome by mounting the sections in styrax dissolved 

 in chloroform instead of in balsam. In this highly refractive medium 

 sections of embryos that have been stained in toto in borax carmine 

 for 24 hours, show the cell-boundaries and nuclei very distinctly, while 

 the disturbing outhnes of the yolk granules are scarcely apparent. 

 After five years these preparations are still unimpaired. 



The curvature of the dorsal surface of the young embryo is very 

 pronounced so that ordinary cross-sections give varying and often 

 deceptive pictures of the pronephros and its duct. This difficulty 

 was overcome by procuring from Yung in Heidelberg a microtome 



1) This arrangement of different stages of some 3 or 4 species 

 or varieties of Petroniyzon in a series for the purpose of illustrating 

 the continuous development of a set of organs might be objectionable, 

 were it not that the various "species" of lamprey are so very closely 

 related (conf. Schneider's remarks on the different European forms 

 [1879, p. 35 — 37] and Gage [1893] on the American species). 



1* 



