RARE FORM OF THE BLASTODERM OF THE CHICK. 391 



Rauber has shown that the phenomena of double monsters 

 may be explained in perfect harmony with the theory of con- 

 crescence. A single illustration must here suffice to show the 

 application of the theory to such cases. 



Figs. 10 and 11 are diagrammatic representations of double 

 formations of the Osseous Fish. In Fig. 10 are seen two inde- 

 pendent formations (a and b), separated by a small portion of 

 the germinal ring (i). Further development leads to the con- 

 dition seen in fig. 11, The concrescence of the lateral halves 

 of the germ-ring carries a and b forward, and brings the por- 

 tions e e together, thus producing a monster that is double in 

 front and single behind. The intermediate portion (i) now 

 forms parts of the mesial sides of a and b. 



The forms here diagrammatically represented have been 

 selected with a view to show that the Vertebrates and Annelids 

 exhibit the same ring-type of development. 



Figs. 1 — 3 = early stages of the Elasmobranch (after 

 His). The manner in which the two halves of the ring are 

 brought together to form the embryo is well illustrated in 

 these figures. 



Figs. 4 — 6 = Salmo (after His). Here the line of concres- 

 cence does not reach to the hind edge of the ring, the two 

 marginal lobes (m 1) seen in figs. 1 — 3 being here represented 

 by a single median lobe. Although the process of concres- 

 cence is thus partially disguised, it becomes evident enough by 

 comparing successive stages as represented in fig. 6. 



Fig. 7 = normal form of the blastoderm of the Chick. 



Fig. 8 = atavistic form of the blastoderm of the Chick, dif- 

 fering from the normal form only in the extension of the line 

 of concrescence to the marginal notch (m n). 



Fig. 9 = Clepsine, showing that in a case of admitted con- 

 crescence the relations of the embryo to the germ-ring are 

 completely analogous to those seen in the blastoderm of the Fish 

 or the Bird. 



Figs. 10, 11 = double formation in the Osseous Fish (after 

 Rauber), 



Fig. 12 — ring-stage of Euaxos (after Kowalevsky). The 



