THE DEVELOPMENT OF THE EARfH-WORM. 2lf 



of them, and abandoning each other rove at their leisure 

 through the albumen. But affairs do not always go on 

 smoothly. There are cases, not at all, rare, in which this 

 strange mode of development leads to true monstrosity ; this 

 happens when the uniting cord does not relax in time to be 

 able to be broken, or when it extends to an abnormal amount. 

 In fact, among perfectly developed worms already hatched 

 double monsters are met with in all grades of concrescence 

 (more or less perfect), from those that are so firmly united 

 along the whole extent of the body that it is impossible to 

 separate them without breaking them to pieces, to others 

 which are hatched coupled together, but only by so thin and 

 frail a ligament that they yet succeed in effecting their sepa- 

 ration, although it may be at a comparatively late period. 

 All, however, have two heads and tAvo tails, two mouths and 

 two ani, well separated ; it appears also that the junction 

 never extends to any internal organ, but always remains con- 

 fined to the epithelial layer of the body-wall. 



The above-described mode of formation of the twin 

 embryos is realised in the great majority of cases, but not 

 seldom embryos are found in other conditions, differing chiefly 

 with regard to the age at which the twins are produced. We 

 have seen above how the differentiation of the layers of the 

 blastoderm begins at one pole while the embryoplastic mate- 

 rial of the other hemisphere is still in an undifferentiated 

 state, but yet that this inequality disappears very soon. 

 There are, however, cases in which a single embryo attains 

 a considerable development before the first rudiment of its 

 companion is formed. I have represented one of them in 

 fig. 5; it is to be understood that this is much further de- 

 veloped than fig. 4. The endoderm has already its peculiar 

 appearance and forms a closed sac ; the germinal streaks are 

 very distinct, although the example lacked any sign of a 

 second embryo if the large cells, which are obviously identical 

 with those of the uniting ligament, do not indicate that a 

 second individual may yet grow out. In fig. 10 is seen a 

 much more advanced embryo, in which, above the opening 

 of the mouth, a small cellular excrescence (z) of a rather 

 irregular form appears, which passes without interruption 

 into the germinal streaks, and is the rudiment of the second 

 embryo. I have found also much further developed embryos, 

 which produced similar buds on the margins of their mouths. 

 On the other hand, I believe the case to be most rare of an 

 egg giving rise to only one embryo, or rather, I should say, I 

 have never ascertained the existence of such a case. It is 

 quite true that sometimes a single worm escapes from a cap- 



