44 THE PINEAL ORGAN 



backward growth from this point, the body and tail of the embryo, during 

 which the remaining section of the germinal ring surrounds the yolk 

 and contributes the lateral and ventral parts of the embryo. As a result 

 of his observations on the development of a series of double-monsters, 

 Fischel emphasizes the limitation of the zone which will give rise to the 

 embryonic rudiment to a relatively small region of the germinal ring, 

 and also the downward (ventral) direction of this growth. He admitted, 

 however, that fusion played an important role in the production of many 

 forms of duplicity. 



Marchand, with whom Dareste may be included, may also be regarded 

 as a supporter of the fusion theory. He held that by far the greater 

 number of symmetrical double-monsters were formed by an extensive 

 union of originally separate embryonic rudiments on a germinal vesicle, 

 although in cases of incomplete anterior doubling he accepted an origin 

 by means of bifurcation. The incomplete Duplicitas posterior he thought 

 most probably arose from two originally separate rudiments (primitive 

 streaks). Schwalbe dissents from Marchand's view and more particularly 

 with reference to the bifurcation theory in connection with the posterior 

 variety. He further points out a group of cases which has to be considered, 

 and which can neither be regarded as belonging to the category of fusion 

 nor in the strict sense of the word to the cleavage theory, and which he 

 refers to under the designation " theory of incomplete separation." Now 

 Ahlfeld defined the controversy between the cleavage and the fusion 

 theories in the following words : " the one assumes that at first a common 

 rudiment was present which in the course of development divided ; while 

 the supporters of the other theory believed that from the very first two 

 separate rudiments can be observed on the germinal vesicle, which in 

 the course of development become united." The theory of incomplete 

 separation coincides with neither of these two theories exactly, and 

 Kaestner contended that the theory of incomplete separation was in reality 

 a modified cleavage theory dressed up in new clothes, and asserted that 

 incomplete double-monsters were not in any particular case one part 

 simple, one part doubled, but were one part completely doubled, the other 

 incompletely doubled ; also that in his opinion those cases in which, 

 instead of there being one primitive streak, two primitive streaks are 

 present, the whole germinal disc to the most distant parts of the embryonal 

 region, into which neither the primitive streak nor the head process has 

 yet penetrated, is already determined or adapted for the development 

 of two embryonal rudiments. Also that all cases of double-monsters 

 which hitherto have been completely examined, proved to be in reality 

 double in all parts ; and where in a general view organs appeared to be 

 simple, these showed on section more or less distinct traces of duplicity. 



