IMPREGNATION OF THE OVUM. 



959 



FIG. 369. Ovum of Scorpaena 

 scrofa. The germinal vesi- 

 cle has extruded a polar body 

 and has withdrawn to the 

 center of the ovum as the nu- 

 cleus; it is being approached 

 by the male pronucleus. 



at a later time during pregnancy, as for example, at the second or the third month 

 (as in a case cited in the Talmud) , then the rare phenomenon of superfetation occurs. 

 This, however, is possible only in the presence of a double uterus and the persistence 

 of menstruation until the time of the second impregnation. Hippocrates explained 

 superfetation as due to independent pregnancies in the horns of the uterus, a 

 condition that according to Aristotle occurs with especial frequency in hares. 

 Superfetation cannot occur in the normal uterus, as a plug of mucus occludes the 

 cervical canal during pregnancy, as Herophilus 

 knew, and in addition from the fact that menstrua- 

 tion usually ceases. 



Hybrids. Impregnation is possible also between 

 related species (horse, ass, zebra; dog, jackal, wolf; 

 goat, ibex; goat, sheep; varieties of the llama; 

 camel, dromedary; tiger, lion; varieties of pheasants; 

 varieties of finch; goose, swan; carp, crucian; varieties 

 of the butterfly). Most of the hybrids thus produced 

 are sterile, chiefly because of a deficiency of developed 

 spermatozoa in the male. The female hybrid, how- 

 ever, may be impregnated by males of the species of 

 either of her parents; for example the mule. The 

 progeny, however, tends to revert in type to the 

 species of the parents. Only a few hybrids are capable 

 of procreation among themselves, as hybrids in dogs. 

 In different species of frogs, the cause of the frequent 

 failure of hybridization is to be found in mechanical 

 obstructions to the penetration of the spermatozoa 

 into the ovum. Only such spermatozoa as are more 

 slender and more vigorous in movement than those of 



the other species are capable of impregnating ova of the latter. Therefore, 

 the possibility of hybridization between two species is almost always one-sided. 

 In some amphibia hybrid fertilization is possible, but development does not take 

 place beyond the first stages. This appears to be due to the circumstance that 

 only a portion of a spermatozoon that has incompletely entered the ovum be- 

 comes active. According to O. and R. Hertwig, hybridization can be more 

 readily effected in echinodermata the more virile the spermatozoa and the 

 feebler the ova. 



In breeding among close blood-relationships (rats) increase of sterile pairings, 

 diminution in the number of offspring, greater mortality among the young and 



marked inability on the part of the 

 mother to nourish them occur. Certain 

 bodily defects and weaknesses appear to 

 be increased. 



Exceptionally the ovum from the 

 ruptured follicle of one ovary may enter 

 the tube of the opposite side, as indicated 

 by the cases of tubal pregnancy and of 

 pregnancy within a rudimentary uterine 

 horn abnormally present, in which the 

 true corpus luteum has been found in the 

 ovary of the opposite side (external trans- 

 migration). In accordance with this 

 observation is the fact that fine granules 

 suspended in water (India ink, etc.), 

 and injected into the peritoneal cavity, 

 penetrate into both tubes, as a result of 

 the action of the cilia, and reach the uterus. 

 In animals ova may also wander through 



the double uterine mouth: out of the one and through the other into the 

 opposite uterine horn (internal transmigration). 



In the maturing ovum the first characteristic change affects the ger- 

 minal vesicle, which divides by mitosis. At the same time it moves to- 

 ward the surface of the ovum, and loses its capsule; its chromatin-fibrils 

 begin to form convolutions and become converted into a longitudinal 

 structure known as the nuclear spindle. At both poles of the spindle, the 

 granular elements of the protoplasmic yolk become collected each into a 



FIG. 370. Ovum of a Starfish (asteracan- 

 thion) with two Extruded Polar Bodies: 

 male and female pronuclei in apposition. 



