v CRUSTACEA ONTOGENY 403 



directed forwards (telson) a deep indentation. The budding zone, from which new 

 segments have formed, borders directly on this indentation. The penultimate 

 abdominal segment (6th) appears, and thus precedes in order of development a large 

 number of segments which lie in front of it. The rudiments of its limbs are already 

 visible. 



Stage H. (Fig. 273 and 278) Embryo with rudiments of ambulatory feet. All 

 the segments of the Crustacean body are formed ; they have developed in succession 

 from the budding zone in front of the telson, so that the anterior and most 

 differentiated are the oldest and the posterior least developed are the youngest 

 (with the exception of the 6th abdominal segment). The budding zone itself has 

 been exhausted in the formation of the segments. The rudiments of the eyes project 

 as spheres. The ambulatory feet have developed. The telson appears deeply forked. 

 The abdomen has become slender, and is so bent forward along the thorax that the 

 telson almost touches the upper lip. The hind-gut has opened into the mid-gut, the 

 epithelial cells of the latter having for some time become swollen by assimilating 

 yolk, and being at this stage columnar. The yolk between the mid-gut and the 

 ectoderm is absorbed, and the wall of the mid-gut is almost in contact with the 

 cephalothoracic ectoderm. The ventral chord with its ganglia differentiates from 

 before backward, having arisen out of paired lateral strands, and a middle strand 

 formed by invagination. The lateral strands are ectodermal thickenings with 

 segmental swellings, which sever themselves from the ectoderm in order from before 

 backward (cf. also Fig. 279). 



Stage J. Embryo with rudiments of abdominal feet. The cephalothoracic 

 shield has developed greatly, its lateral parts project freely as rudimentary branchio- 

 stegites. 



Stage K then follows, with developed eye-pigment and the rudiments of the gills. 

 Thereupon the young Crustacean, which is already tolerably similar in appearance to 

 the adult, is hatched from the egg. The fusing of the anterior thoracic ganglia to 

 form the infra-cesophageal ganglion has begun. The forked telson has become a 

 round plate, and the abdomen resembles that of the adult. The liver is formed 

 almost exclusively by processes of folding of the wall of the mid-gut (cf. Figs. 274 

 and 280). 



FIGS. 275-280. Astacus fluviatilis. Median longitudinal sections through embryos at 

 different stages of development. In Figs. 275, 276, 277, and 279 only the ventral side of the 

 embryo is depicted, in Figs. 278 and 280 the whole embryo in longitudinal section. In Fig. 280 the 

 position of the embryo is the reverse of what it is in the other figures (all Figs, after Reichenbach 



A ^ 



FIG. 275. Stage C. TA, Thoraco-abdominal rudiments ; ene, endodermal invaginatiou (gastrula 

 invagination) ; m, mesodenn ; hi, cephalic lobes. 



