Baer first referred to his findings in 1827 on the 

 double chicken embryo, where only its description had been 

 carried out. He again raised the question of how these 

 monsters were formed, disagreement about which had long 

 existed in the literature on this subject. Haller and 

 Meckel in their own day stated their opinion that double 

 monsters develop by way of union. Burdach later subscribed 

 to this interpretation. Isidore Geoffroy Saint-Hilaire 

 was a more active supporter of the theory of union. Baer 

 decisively raised an objection to this point of view. So 

 he was astonished to find that Barkov, in his thorough work, 

 "Double Deformities among Animals," claimed that Baer's 

 description of the embryo "proved directly the union of 

 separate parts of the accreted embryo." How could it happen, 

 Baer wrote, that, "considering myself a decisive opponent 

 of accretion, I have apparently recognized the accretion of 

 parts of different embryos?" (p. 81) . From the moment of the 

 appearance of Barkov' s work, Baer collected additional data 

 on double monsters, on the basis of which he reconfirmed 

 the accuracy of his previous opinion. 



Speaking of investigations of monsters generally, Baer 

 stated that their development must be studied as exactly 

 as the course of normal development. The achievements of 

 embryology, making the controversy of spermatists and ovists 

 meaningless, was a consequence of that. From elucidation of 

 the primary conditions of the processes of reproduction and 

 development we arrived at the fundamental study of the most 

 formative processes. 



Before turning to the description of many cases of double 

 monsters, Baer outlined his order of presentation. In the 

 beginning he investigated what was formed; later, by comparison 

 with other cases and with normal development, he elucidated 

 the question of how these monsters could be formed. Concerning 

 why these double monsters originate, only suppositions are 

 stated. 



DOUBLE EMBRYOS OF FISH IN VERY EARLY STAGES OF 

 DEVELPMENT. Baer began with the description of two double 

 monsters in perch which he found in 1835 in some eggs obtained 

 from the Neva two days before the investigation. One embryo 

 was double-heated. In the other the division was found in the 



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