110 MAMMALIAN FERTILIZATION 



Fertilization of Mammalian Eggs in vitro 



The literature on the fertiHzation of mammalian eggs in vitro 

 has been reviewed in detail by Chang and Pincus (1951), Smith 

 (1951), and Austin (1951c). The general procedures employed 

 by various workers were to obtain eggs from the follicles or Fal- 

 lopian tubes and to mix them with sperm collected from the epi- 

 didymis or ejaculate in a physiological solution for certain lengths 

 of time. Then the eggs were either cultured in various ways or 

 transferred into recipient animals. From their reviews it seems 

 that until 1951, except, perhaps, for certain experiments of Pincus 

 (1930), there were scarcely any reports that can be credited as 

 definitive, repeatable experiments demonstrating the successful 

 fertilization of mammalian eggs in vitro although such attempts 

 have been made since Schenk (1878). Reported successes were 

 ascribed to artificial activation or to paithenogenetic cleavage of 

 eggs and other accidental artifacts. Because of the frequency of 

 degenerative fragmentation and of parthenogenetic development 

 and the possibility of gynogenetic development, Smith (1951) 

 suggested that the only certain proof of in vitro fertilization would 

 be to observe and photograph the spermatozoon penetrating the 

 vitellus and the subsequent formation and fusion of male and 

 female pronuclei. Because of technical and optical difficulties, it 

 would be too strict a requirement to ascertain the moment of 

 sperm entry. However, the presence of sperm in the perivitelline 

 space and the formation of a second polar body and a male pro- 

 nucleus would be a good indication of fertilization. 



Smith (1951) reported that in the presence of Fallopian tube 

 mucosa a spennatozoon penetrated into the ooplasm of 11 of 35 

 rabbit eggs under a coverslip sealed preparation. Evidence for such 

 penetration was the presence of sperm head within the vitellus, 

 the formation of a male or female pronucleus, or the segmenta- 

 tion of one egg after culture for a longer time. She admitted that 

 sperm penetration was not observed owing to the density of the 

 surrounding corona radiata, and therefore the development might 

 possibly have been parthenogenetic or gynogenetic. It is quite 

 true that when recently shed rabbit eggs are examined under a 



