PRONEPHRIC DUCT IN ELASMOBRANCHS 355 



truth of this the writer examined many series of sections fixed and 

 stained by the most common methods (e.g., corrosive-acetic, 

 Zenker's fluid, picro-acetic, Bouih's fluid, for fixing; and borax- 

 carmine, Delafield's hematoxyUn, alum-hematoxyUn for stain- 

 ing), and found, in every instance that the difficulty of distinguish- 

 ing cell outlines was so great that interpretation was practically 

 impossible. Figures 16 to 24 are drawn from sections of an em- 

 bryo of 6 mm., showing the terminal cell at figure 16 and the an- 

 lage of the duct entirely detached from the ectoderm at figure 24. 

 The sections are consecutive and are intended to demonstrate the 

 same conditions as the previous series, with the additional fea- 

 ture that the duct may be followed from its contact with the ecto- 

 derm behind to the point farther forward where it becomes en- 

 tirely separated from that layer and lies free in the space between 

 ectoderm and the somites. In this series the modification of the 

 ectoderm is very conspicuous, as is also the peculiar mode of at- 

 tachment by means of the cytoplasmic outgrowths from the cells 

 of the tip of the duct. Particular attention is called to figure 22, 

 where the ectoderm seems to be hollowed out for the reception of 

 the anlage, and yet there is not only a limiting membrane, but 

 also a considerable space between the two layers. 



Figures 25 to 35 are from a series of cross sections of an embryo 

 measuring 7 mm., beginning with the terminal cell at the growing 

 tip of the duct and continuing forward until the anlage is practi- 

 cally free of the ectoderm. Prominent among the many interest- 

 ing features in this series is the constant independence of the 

 duct in its growth along the ectoderm. It will be noted that in 

 the entire series there is constantly present a considerable space 

 between ectoderm and duct. The modification of the ectoderm 

 is a marked feature to be particularly noted in figures 28, 31 and 

 35, where the ectoderm is very much affected. Another feature, 

 very clearly exhibited, is the way the anlage is attached to the 

 ectoderm. The medium of attachment (already several times 

 alluded to) is always the product of the mesodermic cells of the 

 anlage and appears to be thrust out not unlike the pseudopodia 

 of an amoeba. It seems to be the same in substance as that 

 composing the processes thrown out by the pronephros before 



