NERVOUS SYSTEM. 



161 



rudiments of the limbs ; these represent the rudiments of the paired 

 ventral ganglia belonging to the individual segments. The consecutive 

 pairs of ganglia are, however, connected by thickened ectoderm-bands, 

 the rudiments of the longitudinal commissures, so that we may 

 regard two longitudinal ectodermal swellings (the " PrimitivwiiJste" 

 Hatschek, Fig. 82, pic) as the first rudiment of the ventral chain of 

 ganglia ; these show segmental swellings (rudiments of ganglia), and 

 are separated by the primitive groove {py). In the later stages of 

 development (Fig. 82 B), in the region of the primitive swellings, 

 the ectoderm is seen to be composed of several layers, an outer 

 one, which noAV is changed into the hypodermis {h) of this region, 

 being separated from the inner layers. The latter now, as lateral 



A 



ft" 



Srri 



3 



Fig. 82. — Development of the ventral chain of ganglia in AstncMn fluviatilis (after Reichenbach). 

 J, cross section through the ttandibular segment of an embrj-o in which the maxillipedes 

 have already appeared. B, cross section through the ganglionic rudiment in a maxillary 

 segment of an embryo in which the maxillipedes have already developed, a", cross section 

 of the second antennae ; bi, connective tissue covering over the inner side of the ganglionic 

 rudiment; cc, ectoderm; en, entoderm; /, bundle of fibres in cross section; gi, ' large 

 ganglion-cells; h, hypodermis; m, invaginated median strand of the ganglionic rudiment; 

 ins, mesoderm ; pr, primitive groove ; pw, primitive swelling of the ganglionic rudiments ; 

 s, lateral strand ; sm, secondary mesoderm. 



strands (s) represent the rudiment of the ventral cord. Reichenbach, 

 on whose description of the development of the nervous system in 

 Astacus our account chiefly rests, was able to prove that, in the 

 formation of each pair of ganglia of the ventral cord, there enters, 

 besides the corresponding portion of the lateral strands, a median 

 invagination (m) ; this is to be traced back to the primitive groove, 

 and is known as the median strand. This agrees with the discoveries 

 of Hatschek in connection with the origin of the ventral chain of 

 ganglia in the Insecta, 



M 



