ii GENERAL EMBRYOLOGY 71 



and half-grown forms. Now what does this mean ? Does each 

 double segment in the full-grown Millepede represent two 

 segments which have become fused together, or is each double 

 segment, so called, a real segment resembling the segments 

 present in the other Orders (for instance, Chilopoda), which has 

 grown an extra pair of legs ? Both these views have been 

 advocated by distinguished naturalists. Neither of them is, in. 

 my opinion, quite right when viewed in the light cast on the 

 subject by recent investigations into the life history of the 

 Chilognatha. 



A close examination into the minutiae of the growth of the 

 different organs has shown us that the double characters ' of the 

 double segments are more deeply seated than was imagined. 

 The circulatory system, the nerve cord, and the first traces of 

 segmentation in t^e mesoblast all show this double character, 

 and the only single part about the segment is the broad plate 

 covering the segment. Now in some of the most ancient of the 

 fossil Myriapods this broad plate shows traces of a division, as if 

 it were in reality t\vo plates fused together. We have also to 

 consider that the life history of the Chilognatha allows us to 

 believe that the peculiar cylindrical shape of the body shown in 

 the greatest degree in the Julidae is attained by the unequal 

 development of the dorsal and ventral surfaces of the body ; the 

 ventral surface being compressed together till it is extremely 

 narrow, and the dorsal surface, as it were, growing round it till 

 the originally dorsal surface forms almost a complete ring round 

 the body. Taking all this into consideration, we are justified, in 

 my opinion, in concluding that each double segment in the 

 Chilognatha is not two segments fused together, nor a single 

 segment bearing two pairs of legs, but is two complete segments 

 perfect in all particulars, but united by a large dorsal plate 

 which was originally two plates which have been fused together, 

 and which in most Chilognatha surrounds almost the whole of 

 two segments in the form of a ring. 



Again in the Chilopoda we see that a great distinctive feature 

 that separates them from the Chilognatha is the character of the 

 ventral nerve cord, the cord being double and not single, a 

 character connected with the fact that the bases of the legs are 

 widely separated from one another, and not closely approached 

 to each other, as in the Chilognatha. As we before said, a more 



