same rudiments as in birds. The lower jaw is a derivation 

 of the first and second branchial arches. Peculiarities of 

 formation of upper jaws in mammals result because the middle 

 does not extend into a long neck like the bird's beak. 

 Every upper jaw develops an inside crest, which gives rise 

 to the corresponding half of the hard palate and nasal septum. 

 Only after this do the jaws of the mammalian embryo begin to 

 extend. In connection with brain development in the previous 

 stages, the facial parts of sheep and swine are very similar 

 to the human face. 



For the digestive canal, the abdominal cavity of mammals 

 closes as in birds, but somewhat later. At first the intestine 

 is connected with the yolk vesicle by a very wide space, the 

 construction and extension of which forms the vitelline duct 

 of the cutaneous umbilicus. The short and non-differentiated 

 digestive canal divides into parts, quickly elongates and 

 moves aside from the backbone, especially in the middle part 

 as a result of the growth of the mesentery. In this place the 

 intestine forms a projection, into which the vitelline duct 

 falls. As in birds, from the digestive tract develop the 

 salivary glands, organs of respiration, liver, pancreas and 

 urinary bladder. A part of the intestinal loop for some 

 time lies in the umbilical branch. 



Differences in the structure of the digestive organs 

 which are inherent to different families of mammals, appear 

 gradually. Furthermore Meckel saw that in the earlier period 

 of development of ruminants, the stomach is single. Notches 

 appear in it, and then the parts of the stomach become 

 separated. Baer noticed Meckel's mistake here,, pointing out 

 that Meckel, in support of his opinion that embryos of higher 

 animals pass through the stages corresponding to organization 

 of the lower organisms, confirmed that the stomach of man 

 passes, as in ruminants, the stage of the sac divided into 

 parts. 



In the vascular system, the heart is initially similar 

 to birds and has the form of a double-branched canal . Becoming 

 shorter, it gains the beginning of five vascular arches, which 

 are transformed into two aortae. Further features appear 

 which are characteristic of mammals. The ventricle turns more 

 to the right side, which is why both ventricles from the 



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