PRONEPHRIC DUCT IN ELASMOBRANCHS 359 



observed. Figure 48 is from a section of a 20 mm. embryo and 

 shows the duct in contact with {he cloaca. It has not yet broken 

 through the cloacal wall. 



In figure 49, from an embryo of 4.5 mm., the anlage has just 

 begun its growth from the pronephros and is pushing its way 

 along the ectoderm. Figure 50 is the next section where the ex- 

 tremity of the growing tip of the anlage is shown. These two 

 sections illustrate very clearly the origin of the structure from the 

 pronephric body. The terminal cell is elongated into a finger-like 

 process composed of the clear cytoplasm, before alluded to, and 

 seems to be feeling its way, as if it were creeping along the ecto- 

 derm. The phenomena suggest the pseudopodia of an amoeba. 

 This feature is very strikingly brought out in the subsequent 

 figures of frontal sections, figures 51 to 54. These also show that 

 the irregular character of the ectodermic layer is not always due 

 to the presence of the growing tip of the duct, as the peculiar thin- 

 ning of the wall seems to occur all along its course, independent 

 of any influence that could be exerted by the pressure of the grow- 

 ing terminal cell. In figures 55 and 61, the duct is very large, 

 compared with the thickness of the ectoderm, and this dispropor- 

 tion seems to preclude the origin of the thicker duct from the 

 thinner ectodermic layer. 



We come now to the consideration of the significance of the 

 presence of mitotic figures in duct and ectoderm which were 

 thought to be of so much importance by. Riickert and Van Wijhe. 

 In his figure 36 Riickert shows a section with three mitotic figures 

 in the region of the point of union between the ectoderm and duct. 

 As will be recalled, he attributes much importance to the fact 

 that the fusion of the duct with the ectoderm makes the tissue at 

 the point of union much thicker and also lays stress upon the 

 presence within the mass, of numerous mitotic figures. To this 

 argument Carl Rabl long ago made what to the writer's mind 

 is an entirely adequate answer. He claims that only one of the 

 cells in mitosis is to be considered — the one in the ectoderm im- 

 mediately contiguous to the duct. This cell is in metaphase 

 (Miitterstern) and is so placed that one sees it in the direction 



