PRONEPHRIC DUCT IN ELASMOBRANCHS 351 



very close at the point of contact, but as the former elongates, its 

 more proximal portion gradually separates from the latter and 

 lies free in the space between the ectoderm and the somites. It 

 is only at the point where the growing tip of the duct impinges 

 upon the ectoderm, if anywhere, that any contribution by the 

 ectoderm to its growth can take place; and it. is here that the 

 various observers claim to have found evidence of such contri- 

 bution. 



In an embryo Acanthias measuring 4.5 mm., corresponding 

 nearly to No. 18 of Scammon's Normentafeln, the development 

 of the pronephros is well advanced. The growth of the proneph- 

 ric anlage toward the ectoderm is very irregular. The ridge 

 does not extend outward with an even margin but seems to be 

 thrust out in processes, sometimes massive and again in long, slen- 

 der, pointed projections, as seen in figures 6, 7a and b, and 8. 

 When the ridge comes into contact with the ectoderm it does not 

 fuse with it but is merely pressed against it. This fact is demon- 

 strated in figure 6, where the mass has been pulled away from 

 the outer layer in the process of preparation, leaving the limiting 

 membrane intact, but leaving its impress upon the ectoderm. 

 The ectoderm at this stage is a one-celled layer and, while the 

 pronephric anlage has made its impression upon it and it has be- 

 come thinned out, yet it remains intact, as will be seen by its un- 

 broken, limiting membrane. Dr. Gregory ('97) figures several 

 instances where the separation of the two layers has occurred. 

 She thinks that cells from the ectoderm have been torn away with 

 the pronephric anlage. I have examined her figures and am con- 

 vinced, by my own observations in similar cases, that the ecto- 

 derm is intact and that the mistake lay in her inability to follow 

 cell outlines and limiting membranes by the method of prepara- 

 tion employed. "When the pronephric anlage has finally reached 

 the ectoderm the outer cells arrange themselves so that the long 

 axis of the nuclei coincide with the long axis of the anlage. This 

 arrangement is preparatory to the formation of the anterior por- 

 tion of the anlage of the pronephric duct which extends the entire 

 length of the pronephric body on its outer margin. At its caudal 

 extremity the pronephric body is prolonged into the space between 



