82 ADAM SEDGWICK. 



represents a section from a Scyllium embryo, in which the 

 segmental tube has just broken away from the muscle plate ; 

 in a slightly younger embryo, or perhaps in posterior sections 

 of the same embryo, the cavity of the segmental tube {st) 

 would communicate with the ventral dilatation of the muscle 

 plate {nip), at the point where they are in contact in the 

 figure. 



This mode of origin of the segmental tubes of Elasmo- 

 branchii renders the origin of the same structures in the 

 chick less extraordinary than it at first sight seemed. 



I refrain now from the discussions and perhaps hypotheses 

 which this observation on tbe development of the Elas- 

 mobranch segmental tubes suggests. On one point, however, 

 there can be little doubt, viz. that segmental involutions of 

 the peritoneal epithelium which have been described in the 

 Teleostei, Amphibia, Ganoids, Mammals, will have to be 

 given up. At any rate the current statement on this point 

 cannot be accepted without further proof. 



I have made observations on the Teleostei and Sturgeon 

 (Mr.Balfour having kindly put his specimens at my disposal) 

 which tend to show, not, however, with certainty, that in the 

 embryos of these forms at any rate there are no serial invo- 

 lutions of the body-cavity epithelium to form the segmental 

 tubes. The method of development in these forms appears 

 to me to be very much modified, if we continue to regard the 

 development in Elasmobranchs as primitive. 



The Wolffian body in all those animals whose ova have a 

 relatively small food yolk seems to be retarded in develop- 

 ment ; while the head kidney, the relation of which to the 

 rest of the excretory system is not understood, attains an 

 early development and functions as the larval excretory 

 organ. Very possibly a clue to the explanation of the re- 

 tardation in the development of the Wolffian body and of the 

 morphological meaning of the head kidney, in Teleostei, 

 Amphibia, &c., may be obtained by a consideration of this 

 coincidence and others hitherto apparently overlooked. 



I hope in another paper to discuss these questions and some 

 others which have been passed over here, and to describe the 

 development of the anterior part of the Wolffian body in the 

 chick. 



In conclusion,! wish to acknowledge the great obligations 

 I am under to Mr. Balfour. Not only have I to thank him 

 for his kindness in placing all his preparations and specimens 

 at my disposal, but, for what has been far more valuable, 

 the help and encouragement he has given nic through the 

 whole course of this investigation. 



