The branchiate slits in the embryos of higher vertebrates 

 were discovered by Rathke (H. Rathke, Kiemen bei Saugethieren, 

 "Isis", 1825, S. 474-749; Kiemen bei Vogeln, Ibidem, S. 1100- 

 1101) . In the letter cited by Baer, Rathke reported the dis- 

 covery of these structures in human embryos too. At the time 

 of his work in Russia, Rathke published a special monograph 

 on this subject (H. Rathke. Anatomisch-physiologische 

 Untersuchungen uber den Kiemenapparat und das Zungenbein der 

 Wirbethiere, 1832, Riga) (435) . 



(110) In relation to this, Baer noted that in all oviparous 

 lizards and snakes the eggs are laid only, when the urinary sac 

 is so developed, that it can perform the respiratory function; 

 Baer referred to his article about duck-bill. Noch eine 

 Bemerkung iiber die Zweifel, welche...(p. 362 (text)) (Arch. 

 Anat., physiol. , <1827, S. 568-576), in which he joined the 

 opinion of Meckel (against Zhoffru Sent Her) about the true 

 existence of lactation glands in duck-bill. The existence of 

 lactation glands, according to Baer's opinion, does not exclude 

 oviparity, as between oviparity and viviparity there is no 

 principal difference, as he showed in lizards and snakes (437) . 



(111) Baer was interested in the embryology of inver- 

 tebrates in the period of his work as assistant of Burdach. 

 Later on they together discovered that in Crustacea the laying 

 of the embryo takes place on the ventral side and grows in the 

 dorsal side. This discovery was not published in the proper 

 time, thus its priority is usually ascribed to Rathke, who studied 

 Crustacea in more detail. Two small paragraphs by Baer were 

 dedicated to the phenomenon of moulting. One was concerned with 

 the so-called renovation of stpmach in river crayfish, which was 

 nothing other than moulting ("Uber die sogenannte Erneuerung 



des Magens der Krebse und die Bedeutung der Krebssteine" Arch. 

 Anat., Physiol., 1834, S. 510-527). In the other paragraph 

 (" ("Beobachtungen uber die Hautungen des Embryos und die Anwendung 

 Anwendung derselben auf die Erkenntniss der Insecten-Metamorphose" 

 Froriep Notizien, 31, No. 10, 1831, S. 145-154) speech goes 

 about the calf embryo, in which under the skin Baer discovered 

 a second layer of skin; he tried to compare this observation 

 (erroneous by all probabilities) to the phenomenon of moulting 

 in metamorphosis in insects (442) . 



644 



