Embryology. 125 



describe development in cellular terms. Some of the 

 steps in this endeavour are of great historical moment, 

 and must be discussed separately. 



Although Schwann and Schleiden clearly recognized 

 that every multicellular organism, reproduced in the 

 ordinary way, begins its individual life as a Nature of 

 single cell, or, in other words that the ovum the Ovum. 

 is a cell, this momentous conclusion required extension 

 and corroboration. In 1828 Von Baer had discovered 

 the mammalian ovum, and in 1861 Carl Gegenbaur 

 demonstrated that the egg of every vertebrate animal is 

 a single cell. Studies of invertebrates yielded the same 

 result, and the discovery of the egg-cells of plants soon 

 followed. Subsequent research has had nothing to add 

 to this simple but fundamental fact; it has concerned 

 itself with the organization of the egg and with the 

 problem of its origin. 



As far back as 1677 Louis de Hamen or Ludwig 

 Hamm, a pupil of Leeuwenhoek, observed the sperma- 

 tozoa of animals, and Hartsoeker claimed a Nature of 

 priority of three years. This matters little, the Sper- 

 however, for neither understood what he 

 saw. For long afterwards these essential male ele- 

 ments were regarded by many as parasitic animalcules 

 wholly unrelated to development (hence the name 

 "spermatozoa"), while other observers, nicknamed 

 " spermatists " or " animalculists ", believed them to 

 be the earliest stages of the young animal, which found 

 the nourishment necessary for development by entering 

 the egg. Even Von Baer (1835) was inclined to inter- 

 pret the spermatozoa as minute parasites peculiar to the 

 male fluid; Johannes Miiller seems also to have been in 

 doubt; and Richard Owen included them in his article 

 on "Entozoa" (internal parasites) in Todd's Cyclopcedia 

 of Anatomy and Physiology. 



In 1786 Spallanzani showed that the sperms were 

 essential to fertilization, since the filtered fluid was 

 impotent; in 1837 R. Wagner emphasized their con- 

 stant presence in all sexually-mature males; Von Sie- 

 bold demonstrated their presence in the invertebrates; 

 in 1 8^ i Kolliker demonstrated their cellular origin in 



