24 EMBRYOLOGY. 



well as the animal egg became known, there soon arose the actively discussed 

 question, whether the egg or the seminal filament was the preformed germ. 

 Decennium after decennium the antagonistic camps of the ovists and of the 

 animaleulists stood opposed to each other. Those who followed the latter 

 thought they saw, with the aid of the magnifying glasses of the times, the 

 spermatozoa of man actually provided with a head, arms, and legs. The 

 animalculists recognised in the egg only a suitable nutritive soil, as it were, 

 which was necessary to the growth of the spermatozoon. 



In the face of such doctrines there dawned a new period for Embryology, 

 when in 1759 CASPAR FRIEDRICH WOLFF in his doctor's dissertation opposed 

 the dogma of the evolution theory, and, casting aside preformation, laid down 

 the scientific principle that what one could not recognise by means of his 

 senses was certainly not present preformed in the germ. At the beginning, so he 

 maintained, the germ is nothing else than an unorganised material eliminated 

 from the sexual organs of the parent, which gradually becomes organised, but 

 only during the process of development, in consequence of fertilisation. Ac- 

 cording to WOLFF, the separate organs of the body differentiate themselves 

 one after another out of the hitherto undifferentiated germinal material. In 

 individual cases he endeavoured, even at this time, to determine more exactly, 

 by means of observations, the nature of the process. Thus C. F. WOLFF was 

 the founder of the doctrine of epigenesis, which, through the discoveries of the 

 present century, has proved to be the right one.* 



WOLFF'S doctrine of unorganised germinal matter has been compelled since 

 then to give way to more profound knowledge, thanks to the improved optical 

 aids of recent times, and to the establishment of the cell-theory by SCHLEIDEN 

 and SCHWANN. A better insight into the elementary composition of animals 

 and plants was now acquired, and especially into the finer structure of the 

 sexual products, the egg-cell and the seminal filament. 



So far as regards the egg-cell, a series of important works began with 

 PURKINJE'S investigation of the Hen's egg in 1825, in which the germinative 

 vesicle was described for the first time. This was soon (1827) followed by 

 C. E. v. BAER'S celebrated discovery of the Mammalian egg, which had been 

 hunted for, but always without success. Extensive and comparative investiga- 

 tions into the structure of the egg in the animal kingdom were published in 

 1836 by R. WAGNER, who also discovered at the same time in the germinative 

 vesicle the germinative dot (macula germinativa). 



With the establishment of the cell-theory there naturally arose the question 

 as to how far the egg was in its structure to be regarded as a cell, a question 

 which was for years answered in widely different ways, and which even now 

 from time to time is brought up for discussion in an altered form. Even at that 

 time SCHWANN, albeit with a certain reservation, expressed it as his opinion that 

 the egg was a cell, and the germinative vesicle its nucleus; but others, his co- 

 temporaries (BISCHOFF and others), regarded the germinative vesicle as a cell, 



* Historical presentations of the theory of evolution and the theory of 

 epigeuesis, which are worth the reading, have been given by A. KIRCHHOFF 

 in his interesting paper, " CASPAR FRIEDRICH WOLFF. Sein Leben und seine 

 Bedeutung fur die Lehre von der organischen Entwicklung." Jenaische Zeit- 

 tchrift fur Medicin und Naturwissenschaft, Bd. IV., Leipzig, 1868 ; and by W. 

 His, " Die Theorien der geschlecht lichen Zengung." Archiv fur Anthropologie, 

 Bd. IV. u. V. 



