On the first day of incubation, the growth of different 

 parts of the embryo is already regular. Baer noted that the 

 curving of the anterior end of the embryo does not represent 

 a direct consequence of the intensified growth of the spinal 

 plates. Instead, he wrote that "this change depends upon the 

 deeper general foundation which is observed at every moment 

 of embryonic formation to separate it from the surrounding 

 parts of the rudiment and from the remaining egg." 



"The (separation) thus depends: 1) on the growth of 

 the embryo, which grows faster than its base (i.e. the region 

 of the union of the embryo with the rest of the blastoderm — 

 L.B.), as well as 2) on the regions of the initial narrowing, 

 connecting the embryo with the primordial membrane; this 

 narrowing becomes noticeable only in the second day" (I, 10; 

 p. 47 (17)). 



Further development of the spinal plates concludes with 

 the final closure of the slits called by Baer the closure of 

 the back, and their breaking up into separate straight-angle 

 parts, described by Baer as the foundation of the vertebrae. 

 This process, it is now known, does not represent the formation 

 of the vertebral column, but the segmentation of the axial 

 mesoderm into parts called somites. From the somites, or 

 the primary vertebrae as they were named not too long ago, the 

 vertebrae and the hard brain membrane develop, and also the 

 muscles, the connective tissue layer of the skin, the kidneys 

 and the sexual glands. 



Summarizing the processes occurring during the first 

 day of incubation, Baer then described the condition of the 

 embryo at that time. The embryo has the shape of an inverted 

 boat. Only the spinal part is distinct, with some pairs of 

 the primary vertebrae; the abdominal side is still entirely 

 unseparated from the primordial membrane. The separation 

 of the embryo from the neighboring parts of the blastoderm 

 takes place only in the anterior part. From all sides the 

 embryo, which has no defined borders, is joined with the 

 primordial membrane and contains the same layers as the 

 latter. The mucous layer lies freely under the primary 

 vertebrae, the region of which represents the most compact 

 part of the embryo. The serous layer of the blastoderm 

 continues into the smooth surface of the spinal plates. 

 The loose layer of the vascular layer continues from the 



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