FERTILIZATION 21 



head enlarges and is converted into the male pronucleus (Fig. 13 D). The 

 pronuclei, male and female, approach each other and resolve themselves 

 first into a spireme stage, then into two groups of 20 chromosomes. A 

 centrosome, possibly that of the male cell, appears between them, divides 

 into two, and soon the first cleavage spindle is formed (F-H). The 20 

 male and 20 female chromosomes arrange themselves in the equatorial 

 plane .of the spindle, thus making the original number of 40 (I). Fer- 

 tilization is now complete and the ovum divides in the ordinary way, the 

 daughter cells each receiving equal numbers of maternal and paternal 

 chromosomes. 



Fertilization of the Human Ovum. Spermatozoa, deposited in the 

 vagina at coitus, ascend through the uterus and uterine tubes, their 

 course being directed by the downward stroking cilia (p. 12). They 

 probably reach the ampulla of the uterine tube two or more hours 

 after coitus. Here the penetration of the ovum is believed usually to 

 take place about one day after coitus (Mall, 1918, cf. p. 12) although it has 

 never been observed. This conclusion is supported by direct observation 

 on other mammals and by the frequency of tubal pregnancies at this site. 

 Normally, then, the embryo begins its development in the uterine tube, 

 thence passes to the uterus and becomes embedded in the uterine mucosa. 

 Rarely ova may be fertilized and start developing before they enter the 

 tube, but fertilization within the uterus is usually denied. 



Twin Development. Usually but one human ovum is produced and fertilized at 

 coitus. The development of two or more embryos within the uterus is commonly due to the 

 ripening, expulsion, and subsequent fertilization of an equal number of ova. In such cases 

 ordinary, or fraternal twins, triplets, and so on, of the same or opposite sex, result. Iden- 

 tical, or duplicate twins, that is, those always of the same sex and strikingly similar in form 

 and feature, are believed to arise from the fission of the embryonic cell mass, each portion 

 then developing as a separate embryo within the common chorion. The identical quadru- 

 plets of certain armadillos are known to result from the division of a single blastoderm into 

 four parts. Separate development of the cleavage cells can also be produced experimen- 

 tally in many of the lower animals. 



Double Monsters. Occasionally twins are conjoined. All degrees of union, from 

 almost complete separation to fusion throughout the entire body-length, are known. If 

 there is considerable disparity in size, the smaller is termed the parasite; in such cases the 

 extent of attachment and dependency grades down to included twin (fetus in fetu) and 

 tumor-like fetal inclusions. In some asymmetrical monsters the explication is partial, 

 as doubling of the head or legs, All of these terata, like identical twins, are regarded as 

 the products of a single ovum, but with variably incomplete fission, or bifurcation, of the 

 embryonic mass. 



Superfetation. An ovum, liberated by a pregnant woman and fertilized at a later 

 coitus, may develop into a second, younger fetus. This rare condition, often denied, is 

 called superfetation and is not to be confused with striking^ unequal twin development. 

 Until the fourth month of pregnancy superfetation is theoretically possible (p. 237). 



