546 



ZOOLOGY 



SECT. 



archenteron, a complete enteric canal being thus constituted. In 

 the meantime the endoderm cells lining the archenteron grow 

 outwards in a radial direction, ingesting the yolk as they do so, 

 until they take the form of long columns, in contact by their outer 

 ends with the ectoderm (Fig. 153, ft}. 



mcs 



nics 



hi- 



Jbcdni 



stdm 



T ni'.cd 7~ 

 ctbtt 



mt/t 



PIG 453. Sections of embryos of Astacus. A, Nauplius-stage (compare Fig. 452) ; -B. alter 

 ' development of thoracic appendages (c/. Fig. 454). abd. abdomen ; an. anus ; or. brain ; 

 eel. ectoderm ; end. endoderm ; ent. enteron ; ht. heart ; mes. inesoderm ; Mies', splancnnie 

 layer of mesoderm ; mth. mouth ; pcdm. proctodaeum ; stdm. stomodoeum ; th. aoa 

 thoracico-abdominal rudiment ; v. nv. cd. ventral nerve-cord. (From Korscnelt an 

 Heider, after Reichenbach.) 



The thoracico-abdominal rudiment soon begins to increase 

 rapidly in length, but, being enclosed in the egg-membranes, it 

 grows not backwards but forwards, being in fact folded upon the 

 anterior part of the body in much the same way as the abdomen 

 of the adult during extreme flexion. 



