trunk region. The division of the cephalic end of the first 

 embryo was incomplete, since there were two mouths and four 

 eyes but only two ears (Figure 34, A) . In the second embryo 

 the posterior part of the body and tail was single and all 

 the anteriorly situated parts were double. Both these cases, 

 in Baer's opinion, totally excluded the possibility of regard- 

 ing them as a result of the union of two individuals: from 

 the moment of the appearance of the embryo not more than one 

 day elapsed; thus during this short period the remaining 

 undoubled parts could not have disappeared. In addition, the 

 embryos were situated close to the yolk. They could not have 

 been any closer in order to unite With each other. In an 

 embryo with a partially doubled head, it can be assumed that 

 the splitting occurred in a very late stage. In the second 

 embryo the bodies were situated at an angle of 120°, indicating 

 that the first foundation of the embryo, namely the primary 

 streak, was divided from the front. The unusual width of the 

 ovum supported this observation. A double embryo appeared 

 after the division of the primary single rudiment. In connec- 

 tion with the description of these cases of double monsters 

 in perch, Baer referred to examples of double embryos in fish 

 described in the literature, beginning with Aldrovandi's 

 MONSTRUORUM HISTORIA (1642) and ending with Heusner's 

 "Double Monsters in Perch and Other Fishes". ^ 



DOUBLE CHICK EMBRYO AT THE BEGINNING OF THE 

 THIRD DAY. In this section Baer gave a more detailed 

 description of a divided chick embryo, the investigation of 

 which was illustrated in 1827 (Figure 34, B) . Here Baer 

 turned his attention to the reciprocal situation of the heads 

 of the double embryo, excluding, in his view, the possibility 

 of the union of two formerly divided embryos. He considered 

 especially conclusive the reasons relating to the situation 

 of dorsal and mainly ventral plates of both partners. "The 

 ventral plate of one of them passed into the convexity of the 

 blastoderm without interruption and without evidence of union 

 in the ventral plate of the other. More exactly, one and the 



5. Heusner, "Descriptio monstrorum avium, araphibiorum, 



piscium." Diss, inaug. Berol. (1824) (cited by Baer) . 



457 



