ORIGIN OF ORGANS. 53 
In other instances, it may arise as a split in a solid mass 
of mesoblast which has been itself produced by delamination 
or by polar ingression. This origin is called schzzocelic. 
A third origin of the coelom is found in the case in which 
the mesoderm arises by multipolar ingression. In this case 
the ingressive cells arrange themselves in two layers to en- 
close the ccelom, which is thus a transformed part of the 
archicoele. Hence this origin is called archicelic, 
The same methods of origin for the archenteron of 
diploblastic larvee can be made out. It will, however, be 
clearly seen that the origin of the layer itself (hypoblast or 
Fig. 20.—THE ORIGIN OF AN ORGAN. 
The upper row shows the cytic origin by single (dark) cells detached from 
parent layer (light). The middle row “shows the dermic origin and the lower 
the tomic, 
mesoblast), and not that of the cavity, is the important con- 
sideration. We must regard the primary layers of hypoblast 
and mesoblast as ovgans, and as such they arise according 
to circumstances in any of the ways in which an organ may 
arise. These may be conveniently generalised as follows :— 
1. Asa number of detached cells from the parent layer 
(cydzc). These may be diffused or localised in their origin. 
2, As a layer of cells or epithelium detached from the 
parent layer (dermic). 
3. As ahollow sac of cells invaginated from the parent 
sac (tomic). 
Organs originating in thes? ways from the three 
primary layers form together the complex organisms found 
