278 VARIATION AND MONSTROSITIES. 



An examination of the different stages of the process (Figs. 185, 189) 

 shows that as the neuromere disappears, the corresponding appendages approach 

 each other and unite, fusing at the base first and at the tip last. The large un- 

 paired leg thus formed first becomes long, slender, and often coiled and twisted; 

 later, it shortens, becomes smaller and smaller, and finally disappears. 



The process of degeneration may be best understood by a consideration of 

 the normal structure of bilaterally symmetrical animals and the way in which it 

 is produced. This may be illustrated by a diagrammatic mercator projection of 

 its superficial organs. (Fig. 192, A.) Here the line, A. P., represents the 

 cephalo-caudal neural line; each lettered square represents a paired segmental 

 organ, and each transverse row a metamere. The middle section shows five 

 body metameres with the typical arrangement of segmental organs, from the 

 neural series, a. a., to the haemal series, e.e. 



The apex of the figure, m.A.n., illustrates the progressive elimination of lateral 

 segmental organs in the head region, and the predominance of the neural ones. 

 The lower part,Z,.P.7?., shows the order in which the organs arise by apical growth. 

 In this process we may recognize two distinct factors, or two different kinds of 

 growth. One gives rise to a longitudinal series of similar metameres, the new ones 

 always appearing just in front of the apex and behind the one previously formed. 

 The other produces a transverse row of unlike organs, i.e., neuromere, ganglion, 

 leg, sense organ, nephridia, heart, etc., extending from the median line outward, 

 the most highly specialized ones being at the median end of each row. 



The relative age of each organ, and its degree of specialization is therefore a 

 function of its position in relation to these two axes of growth. 



Degeneration takes place in the following manner: the most median organs 

 of the first metamere, a and a, unite to form an unpaired organ, A, which then dis- 

 appears, followed in the same way by c. d. and e. The same thing takes place in 

 all the following metameres, the process in the second metamere being one step 

 ahead of that in the third, and so on. 



If the process proceeds till the first two e's of Fig. 192, A., are united, all the 

 organs within the area, a.m.A.n.a.j will have disappeared, and those that were on 

 the margins of this area will form a median row of unpaired, unlike organs, E.A., 

 arranged from before backward in the reverse order of that on the half metamere. 

 (Fig. 192, B.) Embryos in which this condition is approximately realized are 

 shown in Figs. 185 and 189. The condition is shown in diagrammatic form in 

 Fig. 191. 



This mode of degeneration, therefore, takes place according to a definite law, 

 and creates an entirely new, heretofore unrecognized architectural type. This 

 type is fleetingly represented in degenerating Limulus embryos, and probably in 

 many other segmented animals also. The animals in which this condition be- 

 came established in the adults would form a new class. 



