196 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 until 1827, when Carl 

 Ernst von Baer discovered the mammalian ovum. 



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). The observation that the 

 spermatozoon of the rabbit actually entered the ovum 

 was first made by Barry. 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- 

 ess known as fertilization, by which the zygote or ferti- 

 lized cell receives an equal quantity of essential nuclear 

 (chromosome) substance from each gamete, and therefore 

 from each parent of the individual about to be formed. 



It has already been pointed out that the ovum, not 

 being motile, is in a certain sense passive; the sperma- 

 tozoon, which is motile, active in the process. The 

 spermatozoon is no doubt attracted to the ovum by that 

 elementary characteristic of protoplasm already de- 

 scribed as chemotropism. 



The fertilization is differently effected according to 

 the differing conditions of life. Thus, when the animals 

 are aquatic, the ova and spermatozoa may be discharged 



