3. The abdominal part, composed of: 



1) the closed mucous tube, which forms the internal 

 surface of the abdominal part; 



2) the vascular tube which surrounds the mucous tubes, 

 goes above the muscular tube to the trunk, and 

 adjoins to its lower surface; 



3) the muscular tube which is closed upwards by the 

 trunk ; 



4) the second half of the dermal or skin tube which 

 covers the muscular tube. 



Therefore, with a transverse section, such a figure is 

 obtained: the two opposing tubes of the muscular layer form 

 a figure eight, in the middle of which is the trunk; in the 

 upper circle of the figure eight is a tube of neural material, 

 and in the lower circle is the tube of the mucous membrane, 

 surrounded by the vascular layer which continues to the trunk. 

 The muscular layer forms two tubes— the spinal and abdominal— 

 both of which are covered by the common skin cover. The spinal 

 and abdominal tubes represent primary organs and are initially 

 strictly symmetrical. Deviations from symmetry, observed in 

 some vertebrates, have a secondary character. 



The source of symmetry of the primary, tube-shaped organs 

 is their formation from curved and accreted paired plates. 

 All the paired plates (those of the spinal cord, spinal, 

 abdominal, mesenteric and intestinal) which give rise to the 

 aforementioned tubes, could be converted into two pairs of 

 the main plates, the spinal (dorsal) and the abdominal 

 (ventral). Previously, all these plates formed one general 

 plate composed of heterogenous layers. Earlier the different 

 layers were not recognized. 



Thus gradually the grounds of development occur, "but only 

 in reverse succession" (II, 6 (Ah) p. 90. (63)) With these 

 words, Baer stated his method of embryological investigations. 

 He considered that in the beginning it is necessary to study 

 the general organization of the formed animal, that end to 

 which the long chain of developmental processes leads. Later 

 it is possible to pass gradually along the links of this chain 



382 



