in the vascular layer. By these observations, Baer estab- 

 lished the beginning of a theory of development of the 

 vascular system. This theory gained embryological use 

 much later, and Baer's role in its establishment is not 

 always evaluated fairly. 



Turning to histological differentiation, Baer introduced 

 histological elements. At first the embryo is composed of 

 a nearly homogenous mass, partially consisting of dark or 

 light small globules or vesicles, and partially of a trans- 

 parent formless mass. Individual organs at first are also 

 almost entirely homogenous, and only later do fibres, 

 plates and hollow passages appear in them. Baer stated 

 that "modern anatomists called the study of tissue histology, 

 in contrast to anatomy, or the study of the external form. 

 Therefore, in the embryo, the development of separations 

 into multiformed tissues is called histological differen- 

 tiation, represents not a new formation, but a change in 

 what already exists, particularly by separation of the 

 homogenous into variable histological elements. Histological 

 differentiation usually develops later than morphological 

 differentiation; however they are not completely distinct 

 temporally (II 6(c) 11, pp. 122-123 (92)). 



Blood forms, according to Baer, by a thinning of certain 

 parts of the organism, but the walls of the blood-vessels appear 

 with the movement of the blood. All this occurs at first in 

 the vascular layer and then throughout the embryo. 



The processes of histological differentiation are very 

 distinct in muscle formation. At first they look extremely 

 soft, like unclear ly formulated, fairly thick fibers with 

 alternating widened and narrowed portions. These fibers do 

 not grow from other muscles and do not connect one bone with 

 others, but develop in a formless mass located between the 

 bones. Baer objected to Ham's view, that muscles form from 

 small blood globules in a row. Muscle bundles develop by 

 splitting of the initially developing fibers, as a result of 

 which the latter become thinner. 



Concerning nerve formation, Baer believed that nerves 

 do not represent growths from the neural tube. Only the 

 nerves going to the sensory organs represent a growth from 



389 



