INTRODUCTION. 



While Embryology as a science is of comparatively recent date, recorded 

 observations upon the development of the foetus date back as far as 1600 when 

 Fabricius ab Aquapendente published an article entitled " De Formato Fcetu." 

 Four years later the same author added some further observations under the 

 title, " De Formatione Foetus." Harvey (1651), using a simple lens, studied and 

 described the chick embryo of two days' incubation. Harvey's idea was that 

 the ovum consisted of fluid in which the embryo appeared by spontaneous 

 generation. Regnier de Graaf (1677) described the ovarian follicle (Graafian 

 follicle), and in the same year was announced the discovery by Von Loewenhoek 

 of the spermatozoon. These and other embryologists of this period held what 

 is now known as the prejormation theory. According to this theory, the adult 

 form exists in miniature in the egg or germ, development being merely an 

 enlarging and unfolding of preformed parts. With the discovery of the 

 spermatozoon the " preformationists " were divided into two schools, one hold- 

 ing that the ovum was the container of the miniature individual (ovists), the 

 other according this function to the spermatozoon (animalculists). According 

 to the ovists, the ovum needed merely the stimulation of the spermatozoon to 

 cause its contained individual to undergo development, whereas the animalcu- 

 lists looked upon the spermatozoon as the essential embryo-container, the ovum 

 serving merely as a suitable food supply or growing-place. 



Nearly a hundred years of almost no further progress in embryological 

 knowledge came to a close with the publication of Wolff's important article, 

 "Theoria Generationis," in 1759. Wolff's theory was theory pure and simple, 

 with very little basis on then known facts, but it was significant as being ap- 

 parently the first clear statement of the doctrine of epigenesis. The two es- 

 sential points in Wolff's theory were: (i) that the embryo was not preformed; 

 that is, did not exist in miniature in the germ, but developed from a more or less 

 unformed germ substance; (2) that union of male and female substances was 

 necessary to initiate development. The details of W'olff's theory were wrong 

 in tha-t he looked upon the ovum as a structureless substance and upon the 

 seminal fluid and not upon the spermatozoon as the male fecundative agent. 

 Dollinger and his two pupils, von Baer and Pander, were the next to make 

 important contributions to Embryology. Von Baer's publication in 1829 was 

 of extreme significance in the development of embryological knowledge, for 



