EMBKYOGENY 



223 



the present Mammals or Birds shows everywhere adap- 

 tations which have been brought about by the Mammals 

 and Birds. With regard to the evolution of the type 

 6 Mammal ' or ( Bird ' we learn nothing. 



(c) The embryogeny of the Mammalia is precisely 

 as different from the embryogeny of the Fishes as a 

 completely formed mammal is from a complete fish. 

 Just, therefore, as a complete mammal form can under 

 no circumstances be connected with an adult and 

 completed fish, just so can no single stage of germ 

 development (not even in the so-called germ stage) 

 of the Mammals be connected with an embryonic 

 stage of the Fishes. 



That is a result at which 0. Hertwig has arrived 

 after many years of zealous research, and which Naegeli 

 has already expressed in the renowned sentence : ' In 

 the egg of the hen the species is just as perfectly main- 

 tained as in the hen, and the hen's egg is just as widely 

 different from the frog's egg as the hen from the frog. 

 If this appears otherwise to us this is only because 

 in the hen and the frog many distinguishing characters 

 are obvious, while the distinguishing qualities in the 

 eggs lie hidden therein. If the hen's egg did not contain 

 the entire essence of the species, a fowl could not always 

 arise from it with the same certainty/ 1 



The first part of this citation is also found in 

 Korschelt and Herder's 3 well-known textbook expressed 



1 C. von Naegeli : Mechanisch-physiologische Theorie der Abstammungs- 

 lehre, p. 22. 



2 Lehrbuch der vergleichenden Entwicklungsgeschichte der wirbellosen 

 Tiere, part I, p. 136. 



