1890.] Concrescence Theory of the Vertebrate Embryo. 617 
THE CONCRESCENCE THEORY OF THE VERTE- 
BRATE EMBRYO. 
BY CHARLES-SEDGWICK MINOT. 
(Continued from page 516). 
The Meroblastic Embryo—-Considerations of practical conven- 
ience have led to the custom of distinguishing in the develop- 
ment of meroblastic ova the embryonic from the extra-embryonic 
portions. The distinction is in reality entirely arbitrary, for the 
whole of the ovum is included, morphologically speaking, within 
the body of the embryo. Custom has led to designating the 
two parts as the embryo and the yolk; the student should be 
careful not to allow himself 
to be misled by these terms. EN to oN 
In the laboratory it is a gen- > 
eral practice to remove the 
so-called “ embryo ” from the 
yolk, and in doing this the 
entodermic cavity loses its 
inferior wall, to wit, the ento- 
dermic yolk. Let the rela- 
tions be represented by the 
accompanying diagram, the 
embryo being drawn very 
much too large in proportion 
to the yolk for the sake of 
clearness. Suppose the lay- 
PP y Fic. 1o.—Diagram showing a relations of a 
ers to be cut through on the vertebrate ovum with an embryo in cross-section 
Pr 
lines, x x’; we could then groove ; i EMEN. £ é» mern 
: M arc enteric ca’ , ectodermal 
remove the embryonic por- vein, where. da Scene —— ur dio 
tion. This is whatis actually ° 
done in practice. It is very important to understand clearly that 
the yolk is part of the embryo, and that our sections usually 
represent only a torso. 
