874 A Sketch of Comparative Embryology. (December, 
In order to understand the relation of the other axes, we must 
consider briefly the development of the digestive canal in the 
_Echinoderms, and some bilateral animals. The diagrams in Fig. 
22 show the points which concern us now. The ectoderm 
in the young Echinoderm gastrula forms a little pit, Fig. 22 B, 4, 
near the upper end of the gastrula stomach; the bottom of 
this pit grows onto the wall of the stomach, an opening breaks 
through and the pit and the stomach fogm a continuous canal 
with two orifices, Fig. 22 B’. A plane passed through 
these two openings and through the gastrula axis will divide the 
body into symmetrical halves, a right and left. This plane may 
be called the median plane. It is of course purely ideal, not 
present as a structure of the embryo. In the young mollusk, a 
snail for instance, beside the first ectodermal pit, Fig. 22 C, 4, 
there is formed a second one, and always in such a position that 
the median plane passes through it, while the gastrula mouth 
lies between the two involutions of the ectoderm. The gastrula 
mouth ultimately closes, the two pits become connected with the 
entodermal cavity, their exterior openings forming respectively 
the mouth and the anus, A line passed through these two sec- 
ondary openings represents the longitudinal or antero-posterior 
axis. It must not be imagined that these axes necessarily always 
remain straight, for, on the contrary, they usually depart somewhat 
from the simple form, sometimes very much so, as in the case of 
the spirally twisted snails. These axes mark the distinction of 
dorsal and ventral surface, of right and left sides, of anterior and 
posterior ends or head and tail. In the vertebrates the axes are 
further complicated in a manner which will be studied in a spe- 
cial article, and is therefore passed over here. 
The fifth law is that, however much the weight of an animal 
increases during its development, the ratio of the free surfaces to 
the mass alter but slightly from the ratio established when the 
embryo begins to take food from outside. It is only for convenience 
that I express this law in this precise form—in reality, about it 
our knowledge is scanty and our conceptions vague. According 
to a geometrical principle, when the bulk of a body bounded by 
a simple surface increases, the surface enlarges less than the mass 
—in the simplest case of a cube, the surface increases as the 
square, the mass as the cube of the diameter. If in a cube of 
unit diameter, one unit of surface bounds one unit of mass, then 
