Harris Hawthorne Wilder 431 



The various arrangement of these axes may be in the form of an upright 

 Y, an inverted A or an X, thus forming three convenient classes. 



Kaestner, 98, throws some light on the subject by describing a few 

 actual cases, among them that of a shark's egg (Pristiurus), which had 

 two germinal discs. As the egg was transparent the development could 

 be watched, and it was observed that one of these discs soon surpassed 

 the other in size, and that the latter became adherent to the former by 

 the margin. He reasons from this that only in mammals, because of the 

 rudimentary character of the yolk sac, can two such blastoderms ever 

 hope to develop as separate individuals, but that in others (fishes, rep- 

 tiles and birds) the common yolk must lead eventually to a fusion in all 

 cases. 



It will be seen that the principle common to all of the above theories 

 is that of the secondary fusion of two germs, but an insuperable objec- 

 tion to this lies in the complete bilateral symmetry of the two com- 

 ponents in true double mo7isters (diplopagi), since there is no force 

 to oversee and adjust the two components in the exact relationship neces- 

 sary for this result. That such a fusion may result in the other type 

 of compound monster, that of autosite and parasite, seems most probable, 

 a result to which, in all likelihood, the Pristiurus egg of Kaestner would 

 have attained, had it completed its development. 



II. — -A second class of theories rests upon the supposition of a par- 

 tial FISSION IN A SOMEWHAT ADVANCED EMBRYO, thuS doubling the 



parts affected, and several writers have attempted to prove this principle 

 by practical experiment. Thus Valentine, 77, supposes that he pro- 

 duced a double chick by artificially splitting the " Keim," and Gerlach, 

 83, produced a few very questionable specimens by varnishing the shells 

 of hens' eggs and leaving an unvarnished space in the form of a V 

 or a Y over the blastoderm. 



III. — The RADIATION THEORY of Eaubcr is somewhat fantastic and 

 difficult to comprehend, and possesses in this connection mainly an his- 

 toric value. He postulates in the first place a tendency of the germ-cells 

 to radiate in all directions, to assume " eine strahlige Anordnung," a 

 tendency which leads occasionally to the formation of a " Keimring mit 

 mehreren vorderen Embryonalanlagen, statt ^\*ie gewohnlich, einer 

 einzigen." The bifurcation of the anterior end, as taught by Gerlach, 

 Eauber considers a modified form of radiation, and speaks of a radiatio 

 anterior and a radiatio posterior. This theory, like those of Valentine 

 and Gerlach, supposes a splitting of a somewhat advanced embryo, but 

 adds a possilfie, although very hypothetical, cause. 



