the head and trunk, the latter of which is further divided 

 into the thoracic and abdominal parts. The morphological 

 elements are established very early in the embryo, and the 

 differences between them develop late; much later the 

 morphological segments develop. The morphological elements 

 and segments stand in a different relationship to the 

 particular organs. 



Assuming that understanding of organs is devoid of 

 morphological content, Baer found it necessary to introduce 

 a more complete understanding of morphological elements and 

 segments. Thus he held that the eyeball belongs to one 

 morphological element, while the brain occupies a whole 

 segment, which in turn consists of elements. The liver, 

 regardless of its size, is a product of one morphological 

 element, and the small thyroid gland belongs to two elements. 

 In adults, the breakdown of vegetative organs into morpholog- 

 ical elements is not noticeable. The younger the animal, 

 the more this division is obvious; thus, the branchiate slits 

 with their five vascular arches undoubtedly relates to the 

 division of the throat cavity into five parts. In the 

 arthropods there is an obvious breakdown of the whole 

 intestine. 



Baer considered this morphological analysis a very 

 important task which had not previously attracted the 

 attention of embryologists. He expressed his belief that 

 it would be possible to explain the factors upon which all 

 the particular properties of animal structure depend. But 

 this has not yet been accomplished. Nonetheless, interest 

 in problems of animal structure, in particular the phenomena 

 of metamerism which is characteristic of contemporary 

 morphology (investigations of A. N. Severtsov, B. S. Matveev, 

 D. P. Filatov, P. P. Ivanov, N. A. Livanov, V. N. Bekhemishev, 

 and others), bears witness to Baer's insight. 



The actual properties of each primary organ are determined 

 by the character of its morphological division. In particular, 

 cored growths or protrusions form from the neural or intestinal 

 tubes, but the other primary organs only form compact growths. 

 The cored or hollow organs such as the heart and blood vessels 

 can develop not only by means of protrusion (morphological 

 differentiation) , but also by means of histological differen- 

 tiation, particularly due to the develppment of hollow passages 



388 



