DOUBLE EMBRYOS. 



283 



Where do the new halves come from, and by what processes of growth are they 

 formed ? There is no evidence of the existence of special formative material along 

 the median line where the new parts are forming. This is especially clear when 

 the process begins at a late period. The old halves are then quite distinct from 

 the new, and there seems to be no way open to explain the origin of the new half 

 of a segment by lateral budding, or by regeneration, or by growth from the cor- 

 responding old one. We might perhaps infer that the new neuromeres in Fig. 

 187, A.B. come from a kind of regeneration of the old one, but that could not 

 possibly be the case with any of the new organs lateral to the neuromere, such as 

 the appendages, sense organs, and the margin of the mesodermic area. 



Fig. 188. — Double embryos of Limulus, accompanied by median fusion and cephalo-cauded degeneration; 



cam X 16 1/2. 



A comparison of double embryos in various stages shows that the sequence in 

 the production of new organs is as follows : The most anterior metameres are 

 formed first. Each new organ of a metamere first appears as a single organ com- 

 mon to both embryos and having a normal position for each. Additional organs 

 are formed in the same way, in the order of their arrangement on the metamere. 

 For example, the organ nearest the median line is formed first; this then divides 

 into two, and the one lateral to it appears between them as a single organ common 

 to both embryos; this divides, and the next one appears in the same place, till 

 all the organs of a given metamere are formed. (Fig. 187, A.B.) The same 

 process takes place in the next posterior metamere, but it is always one step be- 

 hind that in the metamere in front of it. 



An embryo that has nearly completed its division, as shown in the diagram 

 (Fig. 190), presents a row of median unpaired organs which follow the same 

 order in a cephalic direction that they do in a lateral direction on the metamere, 

 namely a.b.c.d.e. 



The successive eruption of new organs along this median line, and the man- 

 ner in which they divide and move away from it to right and left, is so entirely 

 different from what we have been accustomed to see that it is very impressive. 



