876 A Sketch of Comparative Embryology. (December, 
erly called, there are many kinds, varying in shape and in the char- 
acter of the cells lining them. They 
a maybe straight or very much elongated 
e and coiled or twisted; they branch in 
Dy Many different ways, but all forms are 
modifications derived from simple pit- 
ee 
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lal 
diolo 
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plolela > 
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lJepiaolcjopje]o] 
gelea 
> ———_ 
IG EEC! 
ola 
del 
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anced 
30 
e Bice 
re a y like invaginations. 
ag ga Dje | The necessity of proportioning the 
[ Be Sia à oo J surface to the mass arises from the 
R a Hna gs 5 fact that it is only through the surface 
F- G A al . S Ma] -that food, water and oxygen can be 
G) A > 5, FKA) we taken in, refuse matters (excretions) 
fed fo) | Wy we | ejected, and sensations from the out- 
Uy Bee side received. Hence when the right 
Wau 
& 
4 © P p . . . - 
AAM ia lÀ ) relation is once established it must be 
; = = permanently kept up. In the grow- 
0/1 es" enet ing embryo the object is so to expand 
u a a 
Fic. ai a Vertical section of the surface that as the bulk increases, 
Macrep ea re the surface is always sufficient not 
fer. 210 di only to supply the cells composing 1t, 
but also the inner and deeper lying tissues. 
We can now understand why eggs with very little yolk are 
hatched very early, to become self-dependent larvæ—it is because 
of their small bulk, which enables a simple surface to answer 
their physiological needs, to digest and breathe enough. Bulkier 
eggs must reach a more advanced development, living the while 
on their own yolk, before they can lead a free life. Let it not be 
thought, however, that any one has ever determined, even ina 
single case, the proportion between the surface and the mass- 
There are reasons for thinking that the proportion varies consid- 
erably in different species, and even in individuals of the same 
species, 
The sixth law is, that in all but the lowest metazoa, there are 
several systems of cavities formed in the mesoderm. The meso- 
derm becomes more important and voluminous as we ascend the 
animal series, and so also do the cavities of the middle layer be- 
come more complex. In many animals there is one large space 
known as the body cavity, but the other spaces are for the most 
part small; such are the organs of the circulation, the blood ves- 
