of the outer part of the tail contains a notochord, nerve-cord, connective tissue, pigment cells, and muscle tissue (Fig. 56, _B_). The arrangement of the muscle fibres is generally very irregular, and the characteristic V-shaped arrangement cannot be detected. In only a few cases have attempts been made to unite two eggs or two very early embryos, although there are a few casual observations[89] in which such a fusion has been observed. The problems that arise in connection with the union of two eggs are full of interest. Each egg has the power of producing an embryo of normal size. If two eggs are united into one, will a single giant organism result, or two organisms? If the former, we must suppose that a new organization is formed of double size. Whether an upper limit of organization exists can only be determined by such an experiment. If two fused organisms result from the fusion of two eggs, it would show the structure of the egg is of such a kind that two organizations cannot readjust themselves into a single one of double size. Moreover, it is important to discover whether any difference exists as to the stage of development at which the union is brought about, for it is conceivable that while a rearrangement is possible at one stage, it might not be at another. [Illustration: FIG. 56.--_A._ Tadpole to show where the V-shaped piece is cut from the tail. _B._ Later stage of same with a new tail-like outgrowth from the anterior end of tail.] It has been shown that two blastulæ of the sea-urchin can be united to form a single embryo. I found (’95) that occasionally two blastulæ stick together and fuse, so that a single sphere of double size is formed. As a rule two gastrulæ and two more or less complete embryos develop from each double blastula, but in a few cases I found that a single embryo may be formed, that shows, however, traces of its double origin. Driesch has more recently (1900) succeeded[90] in bringing about more readily a union of two segmenting eggs or blastulæ, and obtained perfect single individuals from two fused blastulæ. He finds that if the fusion takes place at an early stage the resulting embryo is less likely to show its double origin than when older blastula stages are united. Zur Strassen has also observed giant embryos of ascaris that arise by a fusion of two eggs. Loeb has found that the eggs of chætopterus, which can be made to develop parthenogenetically in certain salt solutions, often stick together and produce giant embryos. CHAPTER X THE ORIGIN OF NEW CELLS AND TISSUES