128 KARL ERNST VON BAER 



groups as does the study of adult structure. The four types 

 of development are thus : 



( i ) The double-symmetrical, which is found in Vertebrates. 

 It is called the double-symmetrical, because in Vertebrates 

 development takes place from a central axis (notochord) 

 in two directions, upwards and downwards, in such a 

 way that two tubes are formed, one above and one below 

 the axis. (2) The second type is the symmetrical, which is 

 shown by Annulates. A primitive streak is formed on the 

 ventral surface of the yolk ; development proceeds symmetri- 

 cally on both sides of the streak. (3) Radiate development 

 is probably typical of the radiate structural type. (4) In the 

 massive type, the development seems to be a spiral one. 



Common to most modes is a separation of the germ into 

 animal and plastic layers, a separation which seems to be 

 conditioned largely by the presence of yolk. A classification 

 based upon embryological characters ought to be applied 

 even to the lesser groups and would here prove itself 

 of service. Embryology, for instance, fully supports de 

 Blainville's separation of Batrachia from true reptiles, 1 for 

 reptiles develop an amnion and Batrachia do not. 



We come now to the sixth and last Scholion. Develop- 

 ment is a true evolution of the special from the general, so 

 runs von Baer's most general law of all. This can be expressed 

 in a slightly different way, and the words which he chooses 

 in the sixth Scholion to express this final and most general 

 result are these : " The developmental history of the 

 individual is the history of the growing individuality in 

 every respect " (p. 263). The greatest modern treatise 

 on embryology ends on a splendid note. One creative 

 thought rules all the forms of life. And more " It is this 

 same thought that in cosmic space gathered the scattered 

 masses into spheres and bound them together in the solar 

 system, the same that from the weathered dust on the 

 surface of the metallic planets brought forth the forms of 

 life. And this thought is nought else but life itself, and 

 the words and syllables in which life expresses itself arc the 

 varied forms of the living" (p. 264). 



Von Baer reminds one greatly of Cuvier. There is 



1 Ih' F organisation i/t-x Aniin<ni.\\ i., p. 1^0, 18^2. 



