In spite of this difference of fish and amphibia from 

 the other vertebrates, the formation of their embryo follows 

 the general scheme for all vertebrates: from the embryo two 

 spinal arches arise, two abdominal layers form, and by the 

 closure of these and others the dorsal and ventral sides 

 of the animal form. 



Further, Baer described the ova and their development in 

 amphibia and fish, and devoted separate published works to 

 the embryology of these two classes of vertebrates.^ 



In the ovum of amphibia, which is present in the ovary, 

 the embryonic vesicle appears early. From the beginning it 

 is large and is present in the middle of the ovum. At the 

 time of maturation it ascends to the surface at the place 

 where the embryonic layer is present from outside, not clearly 

 demarcated from the yolk. The embryonic layer with the 

 vesicle present inside it ascends slightly in the form of an 

 embryonic hillock. In a footnote Baer said that he represented 

 this stage in his Epistota de ovi mccmmaliwn genesi (see 

 Figure 26, XXIV-XXVI) . Covered by a thin layer after 

 maturation, the ova fall into the abdominal cavity, and from 

 there into the funnel of the oviduct. Embryonic vesicles 

 are not found in the ova, which are present in the abdominal 



8. Baer, "Die Metamorphose des Eies der Batrachier vor der 

 Erscheinung des Embryo und Folgerungen aus ihr fur die 

 Theorie der Erzeugung," MULLER'S ARCHIV ANAT. PHYSIOL. 

 (1834), pp. 481-509; "Entwickelungsgeschichte der 

 ungeschwanzten Batrachier," BULL. SCIENT., publie par 

 l'Acad. Imp. d. Sciences de St. Petersb., No. 1 (1835), 

 pp. 4-6, No. 2, pp. 9-10 j "Untersuchungen uber die 

 Entwickelung der Fische nebst einem Anhange iiber die 

 Schwimmblase , " Leipzig (1835) , iv + 42 pp. in quarto; 

 "Beobachtungen uber die Entstehungswei der Schwimmblasen ohne 

 Ausfuhrungsgang," BULL. SCIENT., publie par l'Acad. 

 Imp. d. Sciences de St. Petersb., 1, No. 2 (1836), 

 pp. 15-16; "Uber Entwickelungsweise der Schwimmblase der 

 Fische," FRORIEP NEUE NOTIZEN, 39, No. 12 (1846), 

 pp. 177-180. 



419 



