FUNDAMENTAL RELATIONS OF ANIMALS 77 



the changes which the chicken undergoes in the eg^, has gradually 

 extended to every type of the animal kingdom; and so diligent and 

 thorough has been the study, that the first author who ventured 

 upon an extensive illustration of the whole field, C. E. von Baer, 

 has already presented the subject in such a clear manner, and drawn 

 general conclusions so accurate and so comprehensive, that all subse- 

 quent researches in this department of our science, may be con- 

 sidered only as a further development of the facts first noticed by 

 him and of the results he has already deduced from them.^°- It was 

 he who laid the foundation for the most extensive generalizations 

 respecting the mode of formation of animals; for he first discovered 

 in 1827 the ovarian egg of Mammalia and thus showed for the first 

 time that there is no essential difference in the mode of reproduc- 

 tion of the so-called viviparous and oviparous animals, and that man 

 himself is developed in the same manner as the animals. The uni- 

 versal presence of eggs in all animals and the unity of their structure, 

 which was soon afterwards fully ascertained, constitute, in my 

 opinion, the greatest discovery in the natural sciences of modern 

 times. ^'^^ 



It was, indeed, a gigantic step to demonstrate such an identity in 

 the material basis of the development of all animals, when their 

 anatomical structure was already known to exhibit such radically 

 different plans in their full-grown state. From that time a more and 

 more extensive investigation of the manner in which the first germ 

 is formed in these eggs and the embryo develops itself; how its organs 

 grow gradually out of a homogeneous mass; what changes, what com- 

 plications, what connections, what functions they exhibit at every 

 stage; how in the end the young animal assumes its final form and 



loa Without referring to the works of older writers, such as Regnier de Graaf, Mar- 

 cello Malpighi, Albrecht von Haller, Caspar F. Wolff, Johann F. Meckel, Friedrich 

 Tiedemann, etc., which are all enumerated with many others in Theodor Bischoff, 

 "Entwickelungsgeschichte," in Rudolf Wagner, Handworterbuch der Physiologie . . . 

 (12 vols., Brunswick, 1842-1845), I, 860. I shall mention hereafter chiefly those pub- 

 lished since. Under the influence of Ignatius Bollinger this branch of science has as- 

 sumed a new character. See Carl Ernst von Baer, Ueber Entwickelungsgeschichte der 

 Thiere (2 vols., Konigsberg, 1828-1837), the most important work yet published. The 

 preface is a model of candor and truthfulness and sets the merits of Bollinger in a 

 true and beautiful light. 



^'^''Von Baer, De Ovi Mammalium et Hominis Genesi (Konigsberg, 1827); Jan E. 

 Purkinje, Symbolce ad ovi avium historiam ante incubalionem (Leipzig, 1830); Wagner, 

 Prodromus Historice generationis Hominis atque Animalium . . . (Leipzig, 183ji)i Jind 

 Icones physiologicce (Leipzig, 1839). •''o^"-' \ C ; ■ ''"^TN. 



