Further breakdown of the primary organs leads by- 

 indirect morphological differentiation to the formation of 

 definitive organs. Thus, in the cephalic part of the mucous 

 tube, the entrance is differentiated into the respiratory- 

 system and the entrance into the anterior part of the 

 digestive canal. In the latter, the esophagus is narrowed 

 and serves only for transfering food; the second (stomach) 

 is wide and provides digestion for the food Individual 

 areas of the mucous tube protrude and branch away, forming 

 glands (salivary, liver, and the pancreas) . The same 

 principle is implemented also in the development of other 

 tubes such as the neural tube, in which the anterior and 

 thickens in the form of the brain, and the posterior end 

 narrows to form the spinal cord; individual parts of the 

 brain in turn develop into subordinated parts. 



The processes or morphological differentiation, which 

 occur after primary organ formation, conform to some general 

 regulations. Individualization is implemented gradually by 

 irregular growth, such as narrowing, branching and so on. 

 Morphological differentiation spreads from the inert region 

 of the central line toward the fusing line; this route, 

 recognized also in the development of the most primary organs, 

 Baer called the generating arch. 



The form- generating process does not proceed directly 

 from the central lines to the fusing or closing lines; it 

 differentiates the similar morphological elements one after 

 the other. The entire vertebrate body consists of a combina- 

 tion of such morphological elements. Thus, the vertebra with 

 its upper and lower arch is a morphological element of the 

 bone system; the double ring nerves, with part of the central 

 nervous system, represent a morphological element of the 

 nervous system; the blood- carrying system is also composed 

 of morphological elements. 



Along the longitudinal axis, the morphological elements, 

 such as vertebrae of different regions, are not identical. 

 The group of morphological elements with similar features, 

 such as the neck vertebrae, Baer called a morphological 

 segment. By the principle of dividing it into morphological 

 segments, the whole vertebrate body is divided into 



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