364 THE HISTORY OF BIOLOGY 



earliest stages of development of mammals. He described the follicles named 

 after him in the ovary and believed these to be eggs; when later he discovered 

 eggs in the uterus of a rabbit in a later stage of growth, he supposed that 

 these had been moved thither from the ovary for their further development; 

 he met with an insoluble difficulty, however, in the fact that the further 

 advanced eggs in the uterus were smaller than the follicles, and, moreover, 

 the latter proved to be not very constant, wherefore Haller, who carefully 

 investigated the matter, assumed that the egg was formed out of the follicu- 

 lar fluid through coagulation. By carefully following the development of 

 the egg in dogs, von Baer learnt to know its later stages, afterwards trac- 

 ing its origin back by investigating a series of animals approaching nearer 

 and nearer to the fertilization stage. Here he found the egg to be a minute 

 yellowish cell inside the follicle, after which he was able to continue the 

 study of its progressive development. 



His pioneer work on evolution 

 Besides these studies in mammal embryology von Baer devoted himself to 

 that ancient classical object of study in evolutional history — the hen's egg. 

 He followed its evolution with the utmost care and published the results in 

 the first section of the above-mentioned work tjber Entivicklungsgeschichte, 

 which, besides, summarizes all the then existing knowledge of the subject, 

 thereby becoming a pioneer work on which all subsequent research has had 

 to be based. The latter half of the work is a survey of the embryonic develop- 

 ment of all the vertebrates. Through this book von Baer has created modern 

 embryology, not only as an independent field of research, but also as an im- 

 portant branch of comparative anatomy and a means of proving the affinity 

 of different animal forms. In the embryo of the chicken von Baer discovered 

 the spinal cord, which he identified by comparison with the cord of the 

 selachians. He also showed in its proper light Rathke's discovery of the gill- 

 slits and gill-arches in the embryo. Further, he has explained the cause of the 

 amnion formation — a discovery comparable with the foregoing — and has 

 also given concise accounts of the development of the uro-genital apparatus 

 of the formation of the lungs, of the various stages of development of the 

 digestive canal and the nervous system. And finally he makes his proved 

 ideas a basis for a general evolutional theory, which, it is true, contains a 

 mass of natural-philosophical notions, but on the other hand gives a clear 

 survey of the connexion in evolution and far excels all previous theoretical 

 representations, although, since it is ignorant of the part played by the cells 

 in the generative process of the organism, it cannot be called modern. For 

 the process of fertilization is thus substituted a vague hypothesis in which 

 the idea of a growth over and above the individual plays a conspicuous part: 

 ' ' Zuerst wird die Moglichkeit eines neuen Tieres durch unmittelhares Wachsthum des 

 mutterlichen Korpers gegeben. Es bleibt aber nur Teil. Durch die Bejruchtung wird aus 



