white granules, distinguished by this color from the yellow 

 granules of the fluid. With the maturation of the follicle, 

 the embryonic disk gets closer and closer to the periphery. 

 Finally, in the cumulus and in the embryonic disk it is 

 sometimes possible to see the ovule itself. In all animals 

 the round formed mass of the ovule has a dark center, and 

 the transparent periphery is surrounded by a membrane (membrane 

 corticalis) . The size of the ovule in different mammals is 

 different: the largest are in cows, sheep and swine; they 

 are smaller in the rabbit and dogs; the smallest are in 

 humans. 



It is difficult not to be amazed by the accuracy of 

 Baer's microscopical studies, which he accomplished not by 

 the modern use of paraffin sections and selective stains, 

 but by means of preparations with needles and examinations 

 under the microscope. Comparison of Baer's data with modern 

 ideas about the structure of the Graafian follicle leads to 

 the conclusion that he succeeded in seeing all the constit- 

 uent parts of the mature follicle. As a matter of fact, 

 the terminology has changed only insignificantly. Baer's 

 cellular membrane (indusium) is now called the external 

 theca; the internal layer of Baer's theca, which contains 

 the vessels, is called the internal or vascular theca. The 

 transparent layer of Baer's theca presently is called the 

 hyaline membrane. Baer claimed that the cumulus swims on 

 the surface of the follicular fluid, while now it is known 

 that it represents one unit with the granular membrane 

 forming its thickening. The membrane, directly covering the 

 ovum itself, which Baer called the cortical membrane, is 

 now named the transparent zone. 



Section 5 is called "A Brief Review of the Development 

 of Mammals," since Baer considered it important to compare 

 the development of mammals with the development of other 

 animals. Because the ovule is invisible in the early stages 

 of formation of the Graafian vesicle, and since investigations 

 during that period are very difficult, Baer was limited to a 

 hypothetical conclusion that the ovule exists in the Graafian 

 vesicle before it is possible to detect it. He assumed that 

 the ovule of mammals could be comparable with the Purkinje 

 vesicles of other animals, such as molluscs and worms, in 

 which features preceding the development of the ovum are 

 observable. 



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