vertebrae gradually increases so that by the end of the 

 second day the number ranges from 10 to 12. In the head 

 or cephalic region, the knitting spinal plates leave a 

 wider canal, which indicates the location of skull cavity 

 formation. At about the thirtieth hour, the separation 

 of three divisions of this cephalic expansion corresponds 

 to the frontal brain, the four-hillock ("VIERHUGEL") , 4 

 and the medulla oblongata. On the sides of the posterior 

 part of the frontal expansion, projections form which are 

 the first rudiments of the eyes; at about the thirty- third 

 hour, the frontal end of the embryo assumes a similarity to 

 the head of a fly. 



Baer concluded that the rudiments of the cephalic brain 

 and eyes are hollowed vesicles, formed from an initially 

 homogeneous tube. He asked, "What is the stimulus in that 

 process of development?" The answer required an explanation 

 of the further question, "What preexists in the canal prior 

 to the appearance of the cephalic and spinal brain, and when 

 and how do the central parts of the nervous system appear?" 

 (I 2e, p. 56 (24)). Making sure that the spinal plates 

 and even the brain vesicles do not contain "a compact 

 primary mass," Baer concluded that "originally in the place 

 of the cephalic and spinal brain, there is only fluid in 

 the cavities. Under its effect, the initial bulging out of 

 the eyes occurs" (I 2e, p. 57 (25)). 



The abdominal plates begin closing under the vertebral 

 column; this process takes place slowly and continues during 

 the whole period of incubation. In the anterior part of 

 the embryo, where its body is already closed, the abdominal 

 plates form the lateral walls of the body. Wolff named this 

 belt the abdominal plates, more appropriate than Pander's 

 term of abdominal folds. 



4. (Ed.: It is not quite clear what Baer meant by these 

 parts of the early developmental stage. Today 

 embryologists refer to the 33-hour chick as having 

 a prosencephalon (probably Baer's forebrain) , a 

 mesencephalon, rhombencephalon (perhaps together 

 forming the "Vierhugel"?) , and no medulla as yet.) 



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