umbilical vessels stops completely; the umbilicus closes and 

 isolates the chick from its embryonic appendages. On hatching, 

 the chick leaves its membranes, amnion, and chorion with the 

 shell membrane and the shell. 



The next division of Baer's work, entitled "General 

 Method of Formation of the Bird Embryo," (§ 6) concerns the 

 previously established forms of differentiation: the primary, 

 morphological, and histological. In the organization of all 

 vertebrates, Baer considered essential, not the specialized 

 organs of blood-carrying, nervous or digestive system, but the 

 parts similar for all vertebrates. According to Baer, a gen- 

 eralized understanding such as that of the body layers spread- 

 ing along the entire extension of the body could be useful. 

 These layers lie one above the other, as if they develop each 

 other. They become noticeable in the earliest developmental 

 stages, but can also be recognized in the adult. (101) 



Baer prefaced his scheme of the structure of all verte- 

 brates with the characteristics of primary differentiation. 

 Along the longitudinal axis extends the stem (vertebral 

 column) , above which is the spinal part of the animal and 

 below which the abdominal part of the animal. The spinal 

 part consists of the neural tube, the vascular layer, the 

 muscular layer and skin. All these layers have the form of 

 a tube, which forms the primary organs of the vertebrate 

 animal. If we do not take into consideration the extremities, 

 it is possible to consider that the body of the vertebrate 

 animal forms the following parts or layers: 



1. The firm or solid part, which never extends beyond 

 the surface of the trunk. 



2. The spinal part, composed of: 



1) the closed neural tube; 



2) the muscular tube surrounding it; 



3) half of the dermal tube, covering the 

 muscular tube. 



381 



