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 schizocoelic. 



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 coelom, which is thus a transformed part of the 

 archiccele. Hence this origin is called archicoelic. 



The same methods of origin for the archenteron of 

 diploblastic larvae 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 fdark) 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 organs, 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. As a number of detached cells from the parent layer 

 (cytic). 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 a hollow sac of cells invaginated from the parent 

 sac {tomic). 



Organs originating in thesa ways from the three 

 primary layers form together the complex organisms found 



