166 
ADAM SEDGWICK. 
represents a section from a Scytlium embryo, in which the 
segmental tube has just broken aw^ay from the muscle plate ; 
in a slightly younger embryo, or perhaps in posterior sections 
of the same embryo, the cavity of the segmental tube (.9^) 
would communicate wuth the ventral dilatation of the muscle 
plate (mjo), 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 the development of the Elas- 
mbbranch 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 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, I wish to acknowdedge 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 me through the 
whole course of this investigation. 
