distinct: to the fifth day of incubation the yolk increases 

 in volume, arises to the shell, becomes thin, and its granules 

 become more obvious or distinct. These changes were observed 

 by the Kazan professor Eikhval'd. In the process of develop- 

 ment not only morphological, but also chemical transformations 

 occur, and new chemical substances appear. 



Structural changes in the egg lead to the disappearance 

 of the yolk membrane and to the development of the rudiment. 

 The latter enlarges in size and begins gradually to cover the 

 yolk; the central part of the rudiment becomes the embryo, 

 and the large peripheral region remains thin and has the shape 

 of a pellicle. Baer called it the rudiment pellicle or blasto- 

 derm. It represents a continuation of the embryo, with which 

 it is directly connected, and eventually most of it becomes 

 part of the embryo. The rudiment pellicle contains the blood- 

 carrying vessels which receive the nutritional materials from 

 the yolk and transfer them to the embryo. And thus, as the 

 rudiment becomes enlarged, it divides into two parts dis- 

 tinguished by the external shape but connected by a common 

 vital process: its center becomes the embryo, and the 

 periphery becomes the blastoderm. At the beginning the 

 rudiment lies on the surface of the yolk in the form of a 

 plate, and then, on growing, it gradually covers the yolk 

 and acquires a sac form. Already on the fourth day between 

 the embryo and the underlying sac a narrow communication 

 remains. The vesicle containing the yolk is called the yolk 

 sac, or the intestinal or yolk vesicle. 



In the process of development, the rudiment is divided 

 into two incompletely separated layers. From the surface 

 layer the animal parts of the embryo form, and from the 

 internal layer the vegetative or plastic parts. Therefore 

 Baer named them the animal and vegetative layers. At the 

 same time Baer recalled that this animal layer is nothing 

 other than Pander's serous layer, and the vegetative layer 

 corresponds to Pander's vascular and mucous layers. Pander's 

 terms had been worked out during his time in Wiirzburg and 

 had spread since that time. Baer considered the names 

 vascular and mucous layers appropriate but the designation 

 serous layer inappropriate, because the covers of the embryo 

 form only from its peripheral parts, while the middle gives 

 rise to the most important internal organs. Besides that, 



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