THE 
AMERICAN NATURALIST 

VoL. XXIV. JUNE, 1890. 282. 

THE CONCRESCENCE THEORY OF THE VERTE- 
BRATE EMBRYO. 
BY CHARLES-SEDGWICK MINOT. 
T Reta article, and the one to follow, owe their form to the fact 
that they have been written as chapters in my Treatise on 
Human Embryology. As this work cannot be published for some 
time to come, and as the chapters present certain views which 
are fundamentally different from those currently received by em- 
bryologists, I publish them separately. I believe that the right 
understanding of the early development of vertebrates depends 
upon the acceptance of Prof. Wilhelm His’s view of concrescence. 
This view has not received the attention it deserves, for it is not 
based upon elaborate reasoning, but upon the direct observation 
of the process of concrescence in sundry vertebrates. 
Incidentally it will appear that in my opinion neither Hert- 
wig’s Ccelomtheorie, nor Rabl’s Theorie des Mesoderms, can 
be maintained for vertebrates; both of these theories involve the 
assumption that the vertebrate mesoderm arises as two lateral 
masses. This is true of no vertebrate ; on the contrary there is prob- 
ably hardly another fact in embryology so certainly established 
by innumerable observations as the fact that the vertebrate meso- 
derm arises as a single axial structure. It is only secondarily by 
the down-growth of the medullary groove and the up-growth of 
the notochord, that the mesoderm is divided by the meeting of 
these structures into two lateral masses. As regards Rabl’s 
