CR USTACEA ONTOGENY 



401 



FIG. 272. Astacus fluviatilis, embryo in the Nauplius stage. 

 Stage F. A (above), Rudiment of eye ; I, upper lip ; G, cerebral 

 ganglion ; j, anterior antennae ; grao, ganglion of the segment of the 

 2cl antennae (03) ; gm, ganglion of the segment of the mandibles (in) ; 

 Ta, thoraco-abdominal rudiments ; A, (below) anus (after Reichen- 

 bach). 



Stage E. Embryo, 

 with mandibles begin- 

 ning to form. In the 

 middle of the thoraco- 

 abdominal disc the (ecto- 

 dermal) rudiments of the 

 anus and hind-gut (proc- 

 todseum) appear in the 

 form of an anal pit. 

 The mesodermal elements 

 spread out below the 

 ectoderm. 



Stage F. Embryo in 

 the Nauplius stage 

 (Figs. 272 and 276). 

 The 3 most anterior pairs 

 of extremities (Nauplius 

 extremities) can be dis- 

 tinctly made out. Be- 

 tween the anterior an- 

 tennae the first (paired) 

 traces of the brain appear, 

 and immediately behind 

 there, is a pit-like de- 

 pression, the ectodermal 

 rudiments of the mouth 

 and fore - gut (stomo- 

 dfeum). In the posterior 

 antennal and the mandi- 

 bular segments the rudi- 

 ments of ganglia appear 

 as ectodermal thicken- 

 ings in the same way as 

 in the formation of the 

 brain. The thoraco- 

 abdominal ridge, sur- 

 rounded by a deep fur- 

 row, projects anteriorly. 

 The anus has moved 

 forward on it and reaches 

 the inner side of its an- 

 terior edge, i.e. the later 

 ventral side of the end 

 of the body. The ecto- 

 derm and mesoderm form 

 a budding zone on the 

 thoraco-abdominal ridge, 

 and from this time new 



FIG. 273. Astacus fluviatilis, embryo with thoracic feet beginning to form. Stage H. .A, 

 Eyes ; g, brain and ganglia of the anterior antennae (aj) ; 2 , 2d antennae ; m, mandible ; mx^, mx 2 , 

 anterior and posterior maxillae ; <i-g, thoracic feet, of which ti-t-j are maxillipedes ; ts, rudiment 

 of thoracic shield ; db, abdomen, bent back on the anterior portion of the thorax ; T, telson ; I, 

 upper lip ; go, ganglion opticum (after Reichenbach). 



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