of the Annelids and Vertebrates. 469 



embiyo) the ventral side is always directed downwards, 

 and that here therefore dorsum and ventrum are due to mecha- 

 nical causes, in the same way as the upper and under sides of 

 the leaves of plants. 



It is not necessary, however, to commence an investigation 

 in this direction ; for a little reflection shows that though in 

 eggs which have been laid, as in the case of the frog and 

 birds, the ventral side in the germinal disk is frequently 

 directed downwards, it must in just as many cases (namely in 

 ovoviviparous animals) undergo constant changes of position ; 

 nevertheless no deformities arise, and the type of structure 

 remains unaltered. We cannot therefore speak of a cause 

 acting mechanically which in the different symmetrical animals 

 would always bring the same side downwards. Lastly, it 

 follows from the fact that many animals primitively typically 

 symmetrical, like ourselves and the flat tish, do not have the 

 ventrum directed downwards, that the cause which determines 

 the one or the other side as the ventral side is not dependent 

 upon formative laws acting upon the embryo. The type of 

 development in the various animal forms is independent of 

 the direct influence of their position relatively to tlie surface 

 of the earth ; and it appears to be only the position of the 

 mouth which physiologically determines the ventral side. 



1 can see, therefore, nothing in the theorem that the ventrum 

 is the same morphological region in all animals, but an unproved 

 and incoiTCct dogma. Of course, however, this does not neces- 

 sarily imply what Baer appears to have inferred from my 

 views, that there can be no morphological difference between 

 the ventrum and dorsum in the Vertebrates or in the Articu- 

 lates ; on the contrary, I have accepted this difference just as 

 much as Baer himself. But the existing simple distinction 

 between the two regions does not yet prove that the ventrum 

 is identical in V(}rtebrates and Articulates ; on the contrary, 

 the morphological distinction of the ventrum (or dorsum) in 

 the two classes is proved to me by the perfect identity in the 

 types of their development (evolutio bigemina), and by the 

 almost complete correspondence in the relative positions of 

 nearly all the organs in the two groups to one another (but 

 not in their positions in space). 



Baer has of course made use of some morphological argu- 

 ments, in order to support the proposition that the Articulates 

 have their nervous system on the ventrum of the Vertebrates, 

 and that it is therefore comparable to the sympathetic system 

 of the latter group. He refers first of all to the position of 

 the extremities in the Arthropods ; in them, as in the Verte- 

 brates, these arc curved towards the ventral sido. For thi?; 



