great significance and strived to explain its origin and 

 subsequent fate. His comparative investigations led Baer 

 to believe that the embryonic vesicle exists at the earliest 

 stages of egg formation in the ovary. Thus, in hens he saw 

 the embryonic vesicle in ovarian eggs with a diameter of 

 not more than half an inch (about 1.3 mm); the remaining 

 egg parts, including the yolk, are apparently formed later. 

 (Baer had already stated in his DE OVI his belief in the 

 primary existence of the embryonic vesicle.) 



G. Rathke, studying salmon eggs, 2 objected against 

 Baer and confirmed that the "Purkinje vesicle arises . . . 

 considerably later than the yolk." To resolve this dispute, 

 Baer referred to the other objects investigated by Rathke, 

 the river crayfish and fishes. He reported in a later 

 section of his main work (§ 11, footnote pp. 392-394 (11 Bw, 

 296)) that in the autumn, when the eggs increase in size and 

 acquire a color, it is easy to extract the embryonic vesicles 

 included in the voluminous yolk mass. In immature eggs, 

 containing considerably less and still uncolored yolk, the 

 embryonic vesicles are also seen, but they are smaller in 

 size. Even in the smallest eggs with few granules, the 

 embryonic vesicles already exist. In such eggs, there is 

 also a substance dissimilar to yolk, and a few fluid vesicles. 

 Baer concluded that the yolk of eggs which acquire embryonic 

 vesicles is formed only in the period of egg maturation. In 

 the fish Baer also observed that such egg has a nucleus; 

 however, in younger eggs the nucleus is larger and is 

 surrounded with less substance. 



In the process of incubation, the egg loses weight as 

 a result of evaporation, but during the same period unincubated 

 eggs lose less weight than incubated eggs. Simultaneously 

 with the development of the embryo, the volume of the air 

 chamber increases, and the air present in it contains oxygen 

 which the embryo uses. (99) Due to the loss of water, the 

 egg albumen thickens. Changes in the yolk are especially 



2. Rathke, "Uber das Ei einiger Lachsarten," ARCH. ANAT. PHYSIOL 

 (Martin Heinrich Rathke, "Darstellung der 

 spatern Umbildung," in NEUESTE SCHRIFTEN DER 

 NATURFORSCHENDEN GESELLSCHAFT IN DANZIG, 

 VI, Heft 4.) 



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