616 MARGARET ROBINSON. 



pletely left the ectoderm (see fig. 5), Avhich represents a 

 longitudinal section through part of one of the nerve cords in 

 stage B). In this section there can also be seen the small 

 tritocerebral rudiment as well as the rudimentary ganglion 

 of the maxillula3. This last is being absorbed by the mandi- 

 bular ganglion. 



The tritocerebral rudiment here shows no depression. It 

 is simply a mass of nervous tissue, lying between the anten- 

 nary gauglion and that of the mandible. The ganglion of 

 the masillula on the other hand, does show traces of a 

 depression, although it is fusing with, or rather becoming 

 part ofj the mandibular ganglion. 



It can be noticed that all the appendages (first maxillce or 

 maxillula3 excepted) have grown in size, as also has the 

 clypeus. The mouth has moved a little further back, but 

 still lies in front of the antenna and its ganglion. The first 

 maxillae appear very much flattened between the mandibles 

 and the second maxilla3. 



Behind the second maxillary segment there is a segment 

 definitely marked off from those in front of and behind it, 

 and showing distinctly a pair of ganglia. This is the post- 

 maxillary segment. Behind it there follow three segments, 

 each of which bears a pair of rudimentary legs. These 

 appendages spring from the hindmost region of the segments 

 in Avhich they occur, so that they have the appearance of 

 belonging to the segment behind their own. Heymons (1897) 

 has observed a similar appearance in the embryo of 

 Glomeris, and, what is far more important, he has noticed in 

 the same embryo a post-maxillary segment which bears no 

 appendages. While being firmly of the opinion that there 

 go to the making of the gnathochilarium only one pair of 

 appendages and the hypopharynx, he homologises the post- 

 maxillary segment with the labium-bearing segment in 

 Hexapods, and the second maxillary segments in Chilopods. 

 The knowledge of the existence of a pair of maxilla) in front 

 of the gnathochilarium compels one now to homologise these 

 segments differently. (See Table given above.) 



