1880. | A Sketch of Comparative Embryology. 873 ; 
large amount of nutritive matter. These cells are called the 
mesoblasts, and break up 
into smaller cells which 
form the middle germinal 
layer, Fig, 21... ., The. cut 
represents a longitudinal 
section through the double 
embryo of Lumébricus trape- 
soues, after Kleinenberg. 
In this species the develop- 
ment is unique, for each 
egg normally grows into 
two individuals. The separation begins during segmentation. The 
two embryos are united by a cord of large cells, Fig. 21 x, and 
have at first a common mouth. In the right hand embryo of the 
figure, the large mesoblast lies between the inner and outer lay- 
ers, and has already given rise to a number of cells, es, the 
beginning of the mesoderm. In other cases it has been said that 
the mesoderm arises from th2 ectoderm or the entoderm, but 
nearly every observer is contradicted by some other, therefore it 
would be unprofitable for us to pursue the matter further. Suffice 
it to say that the embryonic mesoderm of the Bilateralia consists 
of a mass of cells, or of several strata when the mass is compact, 
whereas the other two layers are each but one cell thick. This 
difference is always preserved, except in the ectoderm of verte- 
brates, to which we 
shall recur. This is 
our third law. ; 
The fourth law is i 
that the cells are Ñ 
grouped in definite 
relations to certain 
ideal axes or planes. 
The first of these 
axes is the gastrula 
Or dorso-ventral,; it 
alone is clearly indi- iii si 
cated in the Colen- „„ẸlC: a2- Diagrams 1o show the mes of the body: 4 
terata. It is the line and hind gut; a, openi ing 4 pal irk 4, of fore gut; 
which passesthrough ^ 9f hind gut. 
the mouth of the gastrula and the opposite end of the body, Fig, 22 A. 
Fic. f Lumbricus trap- 
HAK aaia section, After Kleinen- 
berg. 
