ANNELIDA 



285 



derm (Figs. 130 B and 131). Also in the parts of the 

 embryo lying further backwards the mesodermal bands 

 advance from the ventral side, to which they were at first 

 confined, to the dorsal side, and thus separate the ectoderm 

 from the entoderm. The separation of the mesodermal 

 bands into primitive segments and the splitting of these into 

 somatic and splanchnic layers occurred even before this 

 (Figs. 129 G and 131). The posterior part of the embryo 



-nnvs 



«3 1^— «eaiWiei " t 



Pig. 131. — Optical longitudinal section of an embryo of Lumbricus oUdus (after 

 Wilson-, from Lang's Lehrbuch), bm, fundament of the ventral nerve cord; e, 

 ectoderm ; en, entoderm ; g, fundament of the aupra-CESOphageal ganglion ; kh, 

 head cavity ; m, mesodermal bands ; md, cavity of the mid-gut ; nb, neuroblast 

 cells ; o, mouth ; pm, parietal (somatic) layer of the primitive segments ; pms, pole 

 cells of the mesoderm ; sh, cavity of the primitive segments ; st, stomodaBum 

 (fundament of the fore-gut) ; iig, suucesophageal ganglion ; vm, visceral Csplanchnic) 

 layer of the primitive segments. 



is greatly distended by the albumen taken into the intestine, 

 and bulges out like a yolk-sac on the embryo, which in the 

 meanwhile has grown longer (Fig. 132 /i). Also in the 

 posterior distended part of the embryo the primitive seg- 

 ments are ultimately formed and grow around the ento- 

 dermal sac towards the dorsal side, so that finally it is 

 entirely surrounded by mesoderm ; thus the most im- 

 portant parts in the development of the worm, as far as 

 regards its outer form, are completed. The anus is formed 

 later by an ectodermal depression at the posterior end of 

 the worm. 



