1895.] Embryology. 185 
“ The annelidan structure of the organism manifests itself at a later 
date than the production of the appendages. The folds between the 
rings pass rigorously between the pairs of appendages. The rela- 
tion of cause and effect is apparently evident. This structure is, 
as the temporary metameric disposition of a part of the mesoderm, a 
result of the presence of limbs on the body and of their distribution in 
pairs placed regularly one behind the other at equal or almost equal 
distance apart. The object is to facilitate movements of the body 
especially flexion, and is of no other importance. The morphological 
value of this segmentation of the body into a metameric series is hence 
most plain ; it is secondary, and not primitive, in spite of its analogy 
to annelids and vertebrates, and it is to be associated with the existence 
and arrangement of the paired appendages.” 
In another place the difference betwceni “e mheétamarisi. in n the two 
groups is put as follows: “ In th division of the 
mesoderm is due to a regular increase of cavities in this layer from one 
end tothe other. The appearance of such spaces is not at all dependent 
on the presence of appendages, for it precedes their origin and takes 
place even when they are lacking. Finally these cavities enlarge 
equally and surround the intestine.” 
As to the Arthropods: “Only a part of their mesoderm assumes a 
metameric appearance, the rest remaining mesenchymatous. This seg- 
mental arrangement is not at all a result of multiplication of cavities, 
but the result of a compact grouping of cells due to an inequality in 
multiplication. The spaces which appear finally in the mesoderm are 
irregular, numerous, and in no way related to the segmental arrange- 
ment. This is dependent on the presence of appendages, since it is 
established after these are produced and so that a metamere lies above 
each appendage. The relation isso intimate as to compel the inference 
of cause and effect. Finally, in no case do the mesodermic cavities 
enlarge in a regular way to surround the intestine.” 
Before the embryo is set free, the hump on the back, due to the yet 
unabsorbed food yolk in the dorsal part of the body cavity, disappears 
on the complete absorption of this yolk. This hump has been described 
as a “dorsal organ,” but it is easily seen to be no organ at all, merely 
the last of the food yolk. x 
M. Roule devotes considerable space to the establishment of his view 
as to the formation of the germ layers in Arthropods and their homolo- 
gies with those of other cælomates. He regards the whole process of 
the spreading of the disc- shaped blastoderm as a process of planula- 
tion. No epibolic gastrula is formed, he claims. The planulation 
