VIII 



AKTHKOPODA 



187 



eventually meet above the heart and separate it from the ectoderm. 

 The primary mesoderm becomes divided into a double series of spheri- 

 cal masses, one pair corresponding to each of the segments into which 

 the thoracico-abdominal rudiment is divided. In these somites cavities 



P 



end.p 



FIG. 135. Transverse section through the region of the heart in an embryo of Astacus 

 fluviatilis in about the same stage as that represented in Fig. 134 B. (After Reichenbach.) 



eiid.p, endodermal plate ; //, heart ; pc, spaces destined to form the pericardium. 



appear which represent the coelomic cavities of Peripatus. At the 

 termination of the thoracico-abdominal rudiment there is a deep in- 

 dentation and the anus is no longer visible ; in other words, the tail 

 is divided into two lobes which are termed the tail lobes or caudal 

 fork. The anus has in fact been pushed forwards till it lies in the 



en 



stem 



FIG. 136. Longitudinal section through advanced embryo of Astacus fluviatilis parallel 

 to the sagittal direction but to one side of the middle line. (After Reichenbach.) 



ah.g, abdominal ganglion ; cer, cerebral ganglion ; coe, coelomic cavities ; end.p, endodermal plate ; 

 ext.m, extensor muscles ; proct, hind-gut ; stom, fore-gut ; th.g, thoracic ganglia ; v.s, ventral sinus. 



notch between these lobes, and it then passes on to the ventral 

 surface by the partial fusion of the two tail lobes above it. Just in 

 front of these lobes there is a crescentic area of rapidly growing cells. 

 This is a second budding zone, and it is to its activity that the 

 increased length of the thoracico-abdominal rudiment is due. 



