162 CRUSTACEA. 



The lateral strands are originally composed of simple embryonic cells. In 

 later stages, however (Fig. 82 } their structure is found to be more complicated, 

 a cross section revealing three constituent parts. The first commencements of 

 the formation of nerve-fibres (f) can soon be made out in the innermost (or 

 basal) portion ; these run as two longitudinal bundles below the lateral strands, 

 and are connected with very fine processes of those cells of the strands which 

 become changed into ganglion cells. Besides these paired bundles of fibres, there 

 is, in the rudiment of every pair of ganglia, an unpaired mass of nerve-fibres 

 w y hich perhaps arises from the median strand and gives rise to the transverse 

 commissures. The lateral strands at an early stage become invested with a 

 layer of mesodermal tissue ; this covering, according to REICHENBACH, repre- 

 sents the neurilemma, and penetrates not only into the ganglia, but even into 

 the central mass of fibres. The appearance of masses of fibres at the inner 

 or basal side of the lateral strands has probably the significance of an onto- 

 genetic recapitulation of a primitive condition, in which the whole nervous 

 system was an epithelial structure, with the masses of fibres developed on 

 its inner or basal side. 



Even in quite early stages, REICHENBACH could distinguish, in the rudiments 

 of the ganglia, larger and smaller cells of varying histological character. This 

 distinction is also evident in the fully developed condition. The larger elements 

 (Fig. 82 B, g) give rise to the so-called large ganglion -cells in the central nervous 

 system of Astacus. Similar large cells were observed by NUSBAUM even in early 

 stages in Mysis. In the later stages, massive accumulations of pigment have 

 occasionally been found in the ventral ganglia ; these are probably deposited in 

 mesoderm-cells. Instances of this are to be found in the ganglion which is 

 connected with the sixth pair of appendages of Crangon (KINGSLEY), and in the 

 thoracic ganglia of Mysis (NUSBAUM). 



REICHENBACH'S view as to the participation of a median invagination in the 

 formation of the chain of ganglia has since received only partial confirmation. 

 NUSBAUM, indeed, observed it in Mysii, and GIIOBBEN thought that it could be 

 assumed for Moina. GLAUS, however, denied that a median invagination took 

 part in the formation of the ventral cord in Branchipus. On the other hand, 

 NUSBAUM recently recognised the presence of the median strand in Isopoda 

 (Oniscus, No. 39), in which group BOBRETZKY (No. 80) and BULLAR (No. 81) 

 described the origin of the ventral cord from an unpaired thickening which only 

 at a later stage divided into symmetrical halves. 



With regard to the development of the peripheral nerves, REICHENBACH 

 (No. 65) and CLAUS (No. 9) have shown that it is probable that these do not 

 originate as outgrowths from the rudiments of the central nervous system, but 

 that they appear as distinct ectodermal thickenings at a time when the whole 

 nervous system is still connected with that layer. The transverse commissures 

 which are double in each segment in Branchipus arise in a similar way, according 

 to GLAUS (No. 9). 



In tracing the development of the brain or supraoesophageal 

 ganglion, we must first study REICHENBACH'S minute descriptions of 

 its origin in Astacus (No. 65). According to this author, the whole 

 central nervous system of the pre-oral part of the body arises in the 

 form of three pairs of ganglia, equivalent to one another arid belong- 

 ing to three separate body-segments (Fig. 83). The most anterior 



