214 BIOLOGY: GENERAL AND MEDICAL 



So strong, however, were the preformationists that 

 Wolff failed to make much impression, and, although a 

 simple investigation might have satisfied any scientific 

 observer of error, the real revival of epigenesis was deferred 

 for nearly a century. 



"In 1672 DeGraaf described the structure of 4}he ovary 

 in birds and mammals, observed the ovum in the oviduct 

 of the rabbit, and repeated Harvey's statement as to the 

 universal occurrence of ova, although he mistook for the 

 ova the follicles that now bear his name" Graafian 

 follicles. In 1827 Carl Ernst von Baer definitely traced 

 the mammalian ovum from the uterus, through the oviduct 

 to its origin in the Graafian follicle in the ovary. 



It is difficult to trace the early discoveries appertaining 

 to fertilization. The fact that it is necessary for the 

 ovum to contact with the seminal fluid, in order that 

 fecundation may take place, seems to date from the 

 time of Swammerdan (died 1685); that the spermatozoa 

 were inseparably connected with it from the time of 

 Hartsoeker (1665-1725). In 1780 Spallanzani artificially 

 fertilized the egg of the frog and the tortoise, and even 

 successfully introduced seminal fluid into the uterus of a 

 bitch, but made the mistake of believing that it was the 

 fluid of the semen that caused the fertilization. This 

 error was pointed out in 1824 by Prevost and Dumas, 

 who found that filtration destroyed the fertilizing power 

 of the spermatic fluid. The observation that the sperma- 

 tozoon of the rabbit actually entered the ovum was first 

 made by Barry in 1843. The first to trace the develop- 

 ment of all the tissues from the primordial germinal cells 

 to the stage of complete evolution seems to have been 

 Theodore Schwann (1839). 



The starting point of embryological development, as 

 seen in the light of present scientific knowledge, has 

 been reached in the chapter upon Reproduction where 

 the germinal cells, early set aside in the gonads, or repro- 

 ductive organs, maturing as gametes, pass through the 

 period of maturation characterized by the chromosome 

 reduction. Following this comes the conjugation proc- 



