848 



SPERM, OVA, AND PREGNANCY 



These investigators obtained normal young 

 after separating the first two blastomeres of 

 a rat embryo and transplanting them into 

 pregnant host females. From the results of 

 these experiments, they concluded that "the 

 rat egg possesses the capacity to satisfy 

 two of the criteria for equipotentiality: (1) 

 each of the first two parts of the egg may 

 form a whole embryo which develops fur- 

 ther than the cleavage stages, and (2) the 

 fusion of two eggs produces one single indi- 

 vidual of large size." More recently, Tar- 

 kowski (1959) destroyed a single blastomere 

 in 2- and 4-celled mouse eggs by piercing 

 them with a micropipette directly through 

 the zona pellucida. The eggs were then trans- 

 ferred to properly timed recipients. Of 175 

 half-blastomeres transplanted, 30 per cent 

 had implanted and appeared to be normal 

 except that they were significantly smaller 

 than the controls. Three females gave birth 

 to a total of 6 young which had developed 

 from the experimental eggs. All of the ani- 

 mals were fertile and subsequently gave 

 birth to several litters. Much more experi- 

 mental work is needed in this area, and 

 perhaps the techniques of transplanting in- 

 dividual blastomeres to the anterior chamber 

 of the eye may open possibilities for fur- 

 ther investigations (Fawcett, Wislocki and 

 Waldo, 1947; Runner, 1947). 



L. THE AGE OF THE EGG AT THE TIME 

 OF FERTILIZATION 



In 1913, Jacques Loeb in his book "Arti- 

 ficial Parthenogenesis and Fertilization" 

 wrote as follows: "The unfertilized mature 

 egg dies in a comparatively short time, 

 which may vary from a few hours to a few 

 weeks according to the species or the condi- 

 tions under which the egg lives. The death 

 of the unfertilized egg is possibly the only 

 clear case of natural death of a cell, i.e., of 

 death which is not caused by external in- 

 juries, and the act of fertilization is thus 

 far the only known means by which the 

 natural death of a cell can be prevented." 

 As studies on the physiology of mammalian 

 gametes are pursued, it is evident that these 

 cells must indeed be listed among those hav- 

 ing the shortest life span in the body. 



The reproductive processes in most mam- 

 mals are so timed that spermatozoa reach 



the site of fertilization and are ready to 

 penetrate the eggs almost immediately after 

 their extrusion from the follicles. 



Developmental defects which result from 

 the over-ripening of gametes before fertili- 

 zation have been studied in greatest detail 

 in invertebrates and lower vertebrates 

 (Gemmill, 1900; Bataillon, 1901; Grave 

 and Oliphant, 1930). Because of their ready 

 availability, the eggs of the fish and am- 

 phibia have been found particularly useful 

 in experimental investigations of this type 

 (Mrsic, 1923, 1930; Witschi, 1952). 



Witschi descril)ed a method whereby frogs 

 were induced to retain their mature eggs 

 within the uteri for varying intervals by 

 separating the females from the males and 

 keeping them in dry containers at room 

 temperature. By subsequent removal of the 

 eggs, by either laparotomy or strijiping, the 

 aging gametes could be fertilized by arti- 

 ficial insemination and their development 

 followed. He found that fertilization and 

 development remained relatively normal in 

 eggs which had been retained for 3 to 4 

 days. However, the sex-determining mecha- 

 nism was affected in that almost 90 per cent 

 males were produced. Similar alterations in 

 the sex ratio have been reported by Mrsic 

 in the aging eggs of the rainbow trout. If 

 the frog eggs were retained for more than 4 

 days without fertilization, they gradually 

 became over-ripe and either failed to be fer- 

 tilized or, if penetrated by a spermatozoon, 

 developed abnormally. After approximately 

 1 week, all of the eggs retained in the geni- 

 tal ducts became unfertilizable. In am- 

 phibia, as in other species, the aging eggs 

 gradually lose their vitality. Witschi's ob- 

 servations are particularly significant in 

 that he followed the development of the 

 over-ripe eggs beyond the stage of meta- 

 morphosis and described a number of tera- 

 togenic effects of widely divergent nature. 

 Some of the developmental abnormalities 

 encountered were polymyelia, Polydactyly, 

 axial duplications (especially in the region 

 of the head), anencephaly, microcephaly, 

 and failure of normal differentiation of vari- 

 ous tissues and organs. 



In evaluating the bases for the widely di- 

 vergent nature of these abnormalities, Wits- 

 chi suggested that they are expressions of an 



