282 



VARIATION AND MONSTROSITIES. 



2. Longitudinal fission is radically different from transverse fission, for 

 the latter is the result of a local concrescence and degeneration of segmental 

 organs, while longitudinal fission consists in the formation of two new halves 

 of an embryo along the median line of one already existing. The formation of 

 the new halves begins at the anterior end and extends gradually backward. The 

 old halves are thus thrust apart and each old half, together with the adjacent new 

 half, makes a new embryo. 



Fig. 1S7. — Limulus embryos, stage H, showing various steps in the formation of double embryos. Double 

 embryos are formed, in all the observed cases, by the generation, beginning at the anterior end, of two new halves 

 between the old ones. If there are five paired organs on each segment, and a is median and e lateral, then a will 

 be the first new organ to appear, and it will appear in the median line of the first segment as an unpaired organ. 

 It divides, and in its place in the same segment will be found an unpaired organ, like organ b. But at the same 

 time a new, unpaired organ, like a, will be formed in the median line of segment number two. At the next division, 

 organ a will be produced in the median line of the third segment, b in the second, and c in the first; organs a and b 

 being now completely formed in pairs in the first segment, and organs b in the second. This process goes on till 

 two complete new halves are wedged in between the old, and two new individuals are produced, each individual 

 consisting of an old and a new half. 



A, and B, Early stages in the formation of the new halves; cam. X 16. C, Later stage; cam, X 15 . D, The 

 left-hand embryo has begun to disappear by median fusion and progressive cephalo-caudad degeneration; cam. 

 X 16 1/2. E, The two embryos have completely separated and the one on the left is disappearing by the char- 

 acteristic method of degeneration; cam. X 16 1/2. F, One embryo normal; the other reduced to a single 

 appendage, and a narrow embryonic area; cam. X 16. Stage H. 



This process may be repeated a second time in one of the new embryos, thus 

 producing three embryos tail to tail, consisting of the two original halves plus 

 four new ones. 



All multiple embryos in Limulus are formed in the above manner, never by the 

 partial union of embryos originally separate. This is shown by the fact that 

 in all these cases the embryos match each other exactly, and always in the 

 same way, which could hardly be the case if two separate embryos had 

 united through accidental contact. 



