2 

 name it still has, the "primary zone"; it represents, in 



his opinion, the structure of the vertebral column and 



defines, by its location. the longitudinal axis of the 



embryo. (93) 



From the primary zone, two longitudinal cylinders 

 develop, called Pander primary folds. In Baer's opinion, 

 these require another name, however, because they do not 

 represent primary formations of the embryo and they do not 

 represent true folds. A transverse section shows that these 

 cylinders at first have internal and external slopes of 

 similar form. Then the internal slope becomes sheer, while 

 the external remains gentle. The tops of the cylinders 

 become sharp, like a blade, and incline to each other above 

 their division by a fissure; they merge so that the fissure 

 becomes a closed tube. Baer called these cylinders "spinal 

 plates or disks," from which develop the axial parts of the 

 embryo, namely the central nervous system, the axial skeleton 

 and the muscles. At the same time, the spinal plates develop; 

 at the bottom of their dividing groove the spinal string or 

 cord develops in the form of a central dark zone, situated 

 along the axis of the embryo. This detection of the spinal 

 cord is considered one of Baer's most important 

 embryological discoveries, as he recognized. He likened the 

 identity of the embryonic cord to the rods which a cartilag- 

 inous fish has throughout its entire life. The spinal cord, 

 Baer said, "is not only an axis around which the first parts 

 of the embryo develop, but also a true scale for the whole 

 body and all its main system" (I, 1 1; p. 44 (15)) \ With a 

 preparation method using the thinnest dissecting needles, 

 Baer could separate the spinal cord from its transparent but 

 firm case and pull it out like a lace. 



2. Baer especially stressed that particularly the control 

 zone, and not two parallel folds as Pander thought, 

 represents the first organization of the developing 

 embryo . 



3. Here and next; Lhe Roman number means the volume, the 

 arabic number and latin letter refer to the paragraph 

 and its subdivision.; the pages are related to the 

 Russian issues of UBER ENTWICKELUNGSCESCHICHTE . 

 (Ed.: references to German edition (p. ^).) 



311 



