190 



INVEETEBEATA 



CHAP. 



two points : (1) the mode of formation of layers, i.e. the differentiation 

 of ectoderm, endoderm, and mesoderm, and (2) the larval history. 



an 



FIG. 139. Two sagittal sections through advanced embryos of Astacus fluviatilis. 



(After Reicheubach.) 



A, stage in which secondary yolk pyramids are complete. B, stage in which the endodermic sac is 

 divided into lobes the rudiments of the liver tubes, ab.a, abdominal artery ; aJ,.g, abdominal ganglia 

 n.g.m, anterior gastric muscle; an, anus (the reference line goes to a point some little distance inside 

 the proctodaeum) ; cer, cerebral ganglion ; end, endodermic sac (mid-gut); H, heart; hep, liver saccule ; 

 /'i/i, labrum; in, mouth; p.i/.m, posterior gastric muscle; proct, proctodaeum (hind-gut); r, rostrum' 

 st.a, sternal artery; stow, stomodaeum (fore-gut); M, telson ; th.alid, thoraoico-abdominal rudiment; 

 th.g, thoracic ganglia ; op.a, opthalmic artery ; yj#, secondary yolk-pyramid. 



FORMATION OF LAYERS 



With regard to the first point, all the fragmentary knowledge 

 which we possess of the early history of other Crustacean eo-w s 

 seems to show that they agree in all essentials with the egoof 

 Astacus as to the mode in which the layers are differentiated. If we 



