436 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
head end of the embryo (figs. 35 to 36, and also figs. 36 to 38) can only be explained as ordinary 
growth by intussusception. If this is so, it is perfectly fair to assume, until the contrary is proved, 
that the comparatively small increase in length which the body receives at the tail end is due to the 
same sort of growth. 
I think myself that if the head end grows by intussusception the tail end does 
also. But if the greater amount' of growth takes place at the head end, does that not 
imply that this is the last part to be formed? The oil-globule being a fixed point, 
the embryo has added to its length between figs. “35” and “38” at the head end the dis- 
tance between n and a", or half its entire length , while only one-sixth of its length was 
added to the posterior end between figs. “35” and “38” (the distance between x and x')\ 
The first part formed by the embryo must necessarily be, according to this view, the 
region (nearly) between x and n in fig. 38, which corresponds to xa in fig. 35. To 
put this more precisely, the fifth and sixth sixths of the embryo are the first parts 
formed, and to this rump is added the whole of the head and anterior part of the 
trunk and the neur enteric region. All this comes from the necessity of considering 
the posterior margin of the blastoderm a comparatively fixed point to make out the 
homology with the amphibian egg. It is very probable that the anterior margin of the 
blastoderm does grow over the yolk more rapidly than the posterior in those eggs in 
which the embryo encircles less than half the circumference — Tachysurus, Salmo , and 
Batrachus , for instance — and it is just as probable that the posterior margin grows over 
the yolk more rapidly than the anterior in those eggs in ivliich the embryo encircles more 
than half the circumference of the yolk. There are intermediate eggs, Stolephorus , 
Serranus,* and most other pelagic eggs, in which the anterior and posterior margins 
of the blastoderm grow over the yolk at an even pace. Since writing the above I find 
that these supposed conditions were actually observed, and Wilson was consequently 
refuted by Byder as long ago as 1882, p. 114. Wilson does not mention another pos- 
sibility than his own observations.t 
Explanations of figures and diagrams. — Figs. “35,” “36,” and “38” are copies of Wil- 
son’s figures bearing the same numbers. The position the blastoderm originally occu- 
pied is shown by the line x-y, and the relation of the embryo to the blastoderm and the 
relative growth of the anterior and posterior margins of the embryonic ring can be 
seen at a glance. Between figs. “ 35 ” and “ 36 ” the anterior margin of the embryonic ring 
has traveled from y to y', and the tip of the head from n to a'. Between figs. “36” and 
“ 38 ” the head end of the embryo travels to a", while the posterior end travels to x". It 
should be noticed that the tip of the head in “36” and “38” retains approximately the 
same position in relation to the center of the blastopore that it does at the beginning 
of its formation in figure 35. During all this time the oil-globule has remained sta- 
tionary. This is Wilson’s explanation of these figures. 
* In Serranus atrarius, as figured by Wilson, the embryo encircles slightly more than half a cir- 
cumference and the posterior margin may travel very slightly faster than the anterior. 
tin this connection, the following from the American Naturalist, January, 1894, is of interest. 
The statement occurs in an abstract of Morgan’s experimental studies on teleost eggs : “ It can 
also be shown by marking the membrane with carmine that the head is a fixed point, the elonga- 
tion of the body being posterior to this. The experiment bears out my observation on the oval 
pelagic eggs of Stolephorus, referred to above and figured and described in the Proceedings U. S. Nat. 
Mus. for 1892 (p. 139, pi. Xii). 
