292 



EMBRYOLOGY 



the head ariges from one wall, or from the still undifferentiated meso- 

 dermal bands. The absence of the mesenteries in the head of Puly- 

 gordius supports the view that the cephalic cavity is not paired, but 

 unpaired, in its origin (Hatschek). 



The difference between head cavity and body cavity vanishes when — 

 as is the case, according to Kl-einenberg, in many Annelids, for instance 



'^.-- 



Fig. ^3i.—A to C, parts of frontal longitudinal sections of the larva of Lopad- 

 orhynchus, showing the splitting off of the muscle-plates (after Klkinknbebo). Is, 

 fundaments of setigerous sacs; cct, ectoderm ; ent, entoderm ; g, ganglionic cells 

 of the larval nervous system; mp, muscle-plates; np, neural plates; s, larval 

 sensory organs. 



in Lopadorhijuckns — a regular splitting of the mesoderm into a somatic 

 and a splanchnic layer does not take place, but the investment of the 

 entoderm is effected by separate cells detached from the mesodermal 



