The Morphology of Cosmobia 431 



(h) The primary cause is a subdivision of the egg material, which 

 develops two formative centers (Bildungszentren). 



(c) The cause may lie in a displacement of the early blastomeres with 

 respect to the norm. Granting, with Driesch, that "the prospective 

 significance of a cell is a function of its position," a disturbance 

 of the normal position, and hence the relationships, of a cell may 

 cause it to develop difEerently. 



(d) All that may be postulated in general, to cover all cases, is that 

 the cause is a division of the egg material. Special cases must be 

 considered by themselves. 



The third and last part of the work, which has not yet appeared, is 

 to have the title "Die Einzelmissbildungen." Here, it is evident, we 

 will find the defective cosmobia in the same bad company to which they 

 have long been assigned: schistosoma, exencephalus, fetal amputations, 

 et hoc genus omne. If, however, as we have every reason to expect, 

 they are treated with the same thoroughness as is shown in the rest of 

 the work, the description will be extremely satisfactory, and of the great- 

 est value to teratologists. 



In a series of papers dealing with certain types of double chicks 

 Kaestner^- has sought to ascertain the causes underlying the doubling 

 and has arrived at results which, although not final, are yet of extreme 

 importance. Of especial interest in connection with the present paper 

 are his investigations of cases of two and three day chicks in which the 

 two components are placed with the heads united and the bodies diverg- 

 ing at a greater or less angle to each other, even to a position in which 

 they form a straight line, these he refers to the series of Cephalothora- 

 copagi. In his fifth paper, which is imfortunately the only one that 

 has thus far come into my hand, he gives a thorough descriptive treat- 

 ment of six early avian cephalothoracopagi. All of these he has sec- 

 tioned, and of two he has prepared wax models, many details of which 

 are reproduced in his plates. To my mind one of the most satisfactory 

 points of his paper is the emphasis which he places upon this method 

 of treatment, especially since I had alread}'^, before the appearance of 

 his work, sectioned a chick embryo quite similar to his No. 5, and pre- 



'-Kakst>-er, S. Doppelbilcluiigen an Vogelkoimscheiben. 5te Mitteilnng. 

 Zugleich eiii Beitrag zur Kenntnis der Doppelbiklungen bei Amnioten im all- 

 gemeinen, besonders der Janusbildungen und der ihnen verwandten. Archlv. 

 fur Anat. und Physiol, 1907. (Four earlier papers on the same general 

 subject in the same periodical for 1898, 1899, 1901, and 1902.) 



