284 



VARIATION AND MONSTROSITIES. 



New organs make their first appearance in the same stage of development 

 a.s the corresponding old ones. Each newly formed median appendage first 

 attains a considerable size, then divides at the apex, the separation gradually 

 extending toward the base. This, it will be observed, is the exact reverse of what 

 occurs when degenerating appendages of the right and left sides unite to form a 

 single median one. 



During the later stages of double embryos the growth of the new halves forces 

 the old ones apart, and the two embryos then swing into a straight line, tail to 

 tail. (Fig. 187, C.) They may then separate, moving tail first in. opposite 

 directions till they lie on opposite sides of the egg. (Fig. 187, E.) 



At any stage in this process of diAdsion, one or both embryos may undergo 

 the typical median fusion and antero-posterior degeneration. The process may 

 go on in one embryo quite independently of that in its mate, but always in the 

 typical manner described for the single embryos. (Figs. 187, 188.) 



V. Triple Embryos. 



Triple embryos are very rare. Their mode of origin is shown by the accom- 

 panying diagram. (Fig. 190, B.) It is assumed that in the beginning a single 

 normal embryo, in the manner already described, gives rise to two embryos, of 



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Fig. 189. — Triple embryos of Limulus, showing extensive median fusion and cephalo-caudad 



degeneration. 



A. Of the three embryos in this egg, A is normal and perfect in everything except the abdomen. B has under- 

 gone median fusion and degeneration, and transverse fission. The cephaHc lobes and first four segments have dis- 

 appeared, except two incompletely fused appendages. The abdomen and the posterior part of the thorax persists. 

 The latter is bounded in front of the fifth pair of appendages by a great fold that extends completely across the 

 median line. The nerve-cord in this posterior remnant of an embryo forms a conspicuous, unpaired ridge. Em- 

 bryo C has undergone extensive fusion and antero-posterior degeneration, nothing remaining but the fused appen- 

 dages of the sixth segment, and a rudimentary abdomen. It is probable that the original embryo divided length- 

 wise, giving rise to A and BC, and the latter then divided, giving rise to B and C, Cam. X 15. 



B. Embryo A has undergone median fusion and transverse fission. The fused appendages of the first four seg- 

 ments are arranged in a single row. The cephalic lobes are narrowed, and covered by a hood-like fold of ectoderm, 

 through which one sees the oesophagus. The marginal fold has grown across the median line in front of the fourth 

 pair of appendages. In front of this fold, and near the median line, are the dorsal organs. Embryo B has degen- 

 erated completely in front of the fused fifth pair of appendages, with the exception of the dorsal organs, which have 

 almost reached the median line. In embryo C, the median fusion and degeneration has progressed still farther, 



or the dorsal organs have fused and also the six pairs of appendages. At the central ends of all three embryos, are 

 paired and unpaired ridges, representing abdominal appendages. Mercator projection; cam. X 15. 



C. A triple embryo in which each individual is reduced to an unpaired dorsal organ, to the last thoracic and 

 first branchial appendages. There are faint indications of cardiomeres and of caudal segments; cam. X 16 1/2. 



