424 GENERAL CONSIDERATIONS ON 



and pass out from there through the efferent (nephridial) ducts 

 {Peripatus, Myriopoda). The whole of that part of the primitive 

 segments that is utilised for the formation of the genital glands 

 fuses with those pairs of primitive segments which yield the efferent 

 ducts (nephridia of the genital segments), the continuity between 

 the genital glands and ducts being thus attained. In the Crustacea, 

 as well as in the Arachnida and in the Insecta, there are secondary 

 conditions of development of the genital organs, which, however, 

 are to be traced back to the more primitive conditions just described 

 that still occur in Peripatus and the Myriopoda. 



In consequence of the great abundance of food-yolk in the eggs 

 of the Arthropoda, only the ventral side of the embryo at first 

 appears in the form of a germ-band ; to this rule, however, there 

 are frequent exceptions. The eggs, as has been mentioned, are 

 occasionally small and have less yolk, which may be (in certain 

 rare cases) traceable to a primitive condition, but in most cases 

 must be regarded as a secondary phenomenon. In these cases, the 

 spherical form of the egg may pass directly into the definitive shape 

 of body. 



The germ-band which, in different forms, occupies a more or less 

 considerable part of the egg, arises partly by the ectoderm-cells on 

 the ventral side of the egg becoming elongated, partly by the 

 appearance beneath it of the two other germ-layers, especially of 

 the mesoderm-bands. Besides this, the band-like thickenings of the 

 ectoderm soon appear near the ventral middle line, representing 

 the rudiment of the ventral chain of ganglia, which, indeed, very 

 soon breaks up into segments. A much widened anterior section 

 of the germ-band very soon becomes distinguished from the primary 

 trunk of the embryo as the cephalic lobes. The trunk soon 

 breaks up into segments, this modification chiefly involving the 

 mesoderm (formation of the primitive segments;, but may also be 

 indicated on the external surface of the germ-band even before the 

 appearance of the primitive segments (Hydrophilus, Chalicodoma). 

 The series of limb-rudiments appear as outgrowths of the surface 

 on each side ; in most Arthropoda, a process of a coelomic sac 

 passes into each of these limbs, so that they at first appear hollow 

 {Peripatus, Myriopoda, Orthoptera, Arachnida, Pantopoda). Even 

 in those forms which, in the adult, have no limbs on the abdomen 

 (Arachnida, Insecta), abdominal limbs are found in the embryo ; 

 the abdomen of the embryo also may consist of a larger number 

 of segments than that of the adult (Araneae), an indication of the 



