In connection with the bending of the anterior end of 

 the embryo, under the head part an increasing deepening of 

 the blastoderm appears in the form of folds. Baer called 

 this formation the "head cap" (Kopfkappe) (I 2h (26)). The 

 displacement of the top of the cap backwards denotes the 

 separation of the head end of the embryo. The deepening 

 of the mucous layer extends, forming the first rudiment of 

 the digestive canal in the form of a blind sac. This sac 

 opens downwards and backwards with a wide opening into the 

 stomach. Baer called this opening the "anterior entrance 

 into the alimentary canal, "^ suggesting his rejection of 

 Wolff's inappropriate term Fovea cavdiaca. 



Between the close anterior ends of the abdominal plates, 

 the vascular layer becomes thick and a granular mass appears 

 in it, forming two lateral projections whose ends are 

 directed backwards. This is the material for the formation 

 of the heart. In the middle of the second day, the central 

 part of the described rudiment becomes transparent; internally 

 its contents become blood, and the external layer becomes the 

 heart wall. At the same time, the central nervous system 

 becomes independent. On the second half of the second day, 

 at the internal surface of the spinal plates, a layer forms 

 a hollow cylinder, the lateral walls of which are thickened. 

 This is the fold of the cephalic and spinal brain. In the 

 head division the future parts of the brain are originally 

 one unclear mass. They then become separated by weak 

 interceptors, which begin dividing into two hemispheres. 

 The canal from the brain to the eye, which is the future 

 optic nerve, remains hollow at this stage. The primary 

 foundation of the ear begins to occur, according to Baer, 

 in the same way as the formation of the eye rudiment, i.e. 

 as a hollow outgrowth from the medulla oblongata. 6 



Turning to the foundation of the vascular system, Baer 

 referred to Wolff's and Pander's observations on vascular 

 growth. Confirming their observations, he remarked that the 



5. The term is kept also in the modern embryology. 



6. Actually, the rudiment of the internal ear (auditory 

 vesicle) is formed from the unlaced deepening of the 

 ectoderm. 



317 



