Later it becomes longer, the ventral becomes rounded and 

 its internal cavity is situated on a lobule (Figure 32, 27) . 

 The first protrusion represents the rudiment of the swimming 

 vesicle, and the second the rudiment of the liver. By the 

 fourth to fifth day the swimming vesicle consists of two 

 parts, an elongated sac and a hollow stalk similar to the 

 branchus of the simple lung. The vesicle, when seen under 

 the microscope or in live fish, still does not contain air. 

 After the fifth day the swimming vesicle fills with air and 

 thereby becomes significantly larger and distinguishable by 

 the naked eye. 



The anterior swimming vesicle in adults is connected 

 with the posterior one and with the auditory organ. It is 

 formed later than the anterior, after four weeks, when the 

 body of the fish is already opaque and its formation is 

 thus very difficult to follow. Nevertheless, on the basis 

 of his direct observations Baer presented an astute dis- 

 cussion of development of the anterior swimming vesicle 

 with the auditory apparatus. The last question is discussed 

 in a special addition to the above-mentioned work of 1835 

 on fish development, in which he classified the swimming 

 vesicles of the different fish in connection with the history 

 of development of those organs. 



Two earlier works, especially illustrating the study of 

 branchial slits and branchial vessels in different vertebrates, 

 serve as an addition to the third part of ttBER ENTWICKLUNGS- 

 GESCHICHTE. They represent a continued essay of the com- 

 parative embryology of vertebrates. 21 The first of these 

 works he began with "Not long ago my dear friend Dr. Rathke 

 (109) wrote me. Finally I found in the human embryo hints of 

 gills, in particular in one six or seven-week-old aborted 

 embryo. From each side there were two gills, the anterior 



21. Baer, "Uber die Kiemen und Kiemengefasse in den Embryonen 

 der Wirbelthiere," ARCH. FUR ANAT . U. PHYSIOL. 

 (1827) , pp. 556-568? "Uber die Kiemenspalten des 

 Saugethier-Embryonen," IBID. (1828), pp. 143-148. 



435 



