GEU.STAGEA—ONTOGEXY 



401 



Fig. 272.— Astaous fluviatllls, embryo in the Nauplius stage. 

 —Stage F. A (above), Rn.liment of eye, ; ;, upper lip ; 0, eerebral 

 ganglion ; ai, anterior antenufe ; gao, ganglion of the segment of the 

 2d antenna; (aa) ; gm, ganglion ot the segment of the mandiblps (m) ; 

 Ta, thoraco-abdoininal rudiments ; A, (below) anus (after Eeichen- 

 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- 

 todfeum) 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 {Ifauplms 

 extremities) can be dis- 

 tinctly made out. Be- 

 tween the anterior an- 

 tennae the first (paii-ed) 

 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 anns 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.- Astaous fluvlatills, embryo with thoracic feet beginning to form.— Stage H. A, 

 Eyes ; g, brain and ganglia of the anterior antenna; (aj) ; a^, 2d antennee ; m, mandible ; mz^, mx^, 

 anterior and posterior maxillse ; ti-tg, thoracic feet, of which ij-ig are maxillipedes ; is, rudiment 

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

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



VOL. I 2d 



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