292 



EMBRYOLOGY 



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

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

 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 Kleinenberg, in many Annelids, for instance 



Fig. 134.— il to C, parts of frontal loncjitudinal sections of the larva of iopod- 

 orJiyncJius, showing the splitting off of the muscle-plates (after Klkinkn bebg). bs, 

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

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

 sensory organs. 



in Lopadorhynchus — 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 



