502 The American Naturalist. [June, 
theory of the double origin of the vertebrate mesoderm, I think 
he would not have advanced that theory, had he not first fallen 
into the error of rejecting Prof. His’s concrescence theory.! 
That the fundamental difference between the mesothelium and 
mesenchym involved in the Ccelomtheorie cannot be maintained 
for vertebrates, I have pointed out elsewhere? This impossibility 
has also been brought out afresh by the investigations of Bonnet, 
Ziegler, Strahl, Rabl, and others. 
As briefly indicated in my article on Segmentation of the Ovum 
(AMERICAN NATURALIST, 1889, June and September), the vertebrate 
diaderm (stage with ectoderm and entoderm only), advances to 
the next stage of development by the concrescence of the two 
halves of the ectental line to form the structural axis of the future 
embryo. The process is somewhat complex, and needs therefore 
to be described in detail, the more so as it has still to be traced 
in mammals. 
Historical Note.—The earliest observations on concrescence to 
form the embryonic axis are, so far as known to me, those of 
Rathke on leeches? Nine years later, Kowalewski (Mem. Acad. 
Sci SE Petersbourg, 7th Ser, XVI. 1871), recorded its occur- 
rence among insects. Its recognition as a vertebrate mode of 
development we owe to the brilliant investigations of W. His; 
in his first paper, 26, he describes very accurately and clearly 
the process of concrescence in the salmon; in his second paper, 
27, he describes concrescence in the sharks, and in his third 
paper, 28, he discusses again the general bearing of his re- 
sults. Semper in his great work on the relationship of annelids 
and vertebrates, 54, was the first to make a direct comparison 
of the processes of concrescence in annelids, insects, and verte- 
brates. Unfortunately Balfour entirely failed to grasp the new 
conception, and by expressing himself very decidedly against it, 
Comp. Embryol. IL., 306-308, led many embryologists to discredit 
1 Rabl’s criticisms of His are very much to be regretted. If the former always ob- 
nAi with the precision and accuracy of the latter, he will add to his already high repu- 
2 Buck's Reference Handbook, Med. Sci., IIL, 176. 
3 Rathke and Leuckart, 
Beitrage zur Entwickelungsgeschichte der Hirudineen, 
Leipzig, 1862. 
