172 READINGS IN BIOLOGICAL SCIENCE 



propulsion.The total period required by human spermatozoa in reaching 

 their destination is unknown, but it cannot be more than a few hours at 

 the most. There is no good reason for beheving that the duration of fer- 

 tilizing capacity of the sperms extends beyond a day or two. 



The penetration of the ovum by the spermatozoon and the resulting 

 fusion of their respective nuclei, constitutes the process of fertilization. In 

 practically all animals fertilization also supplies the stimulus that starts the 

 ovum dividing. The meeting and union of the human cells is believed 

 usually to take place in the upper third of the uterine tube. 



TWINS AND TWINNING 



The frequency of multiple births varies considerably among different 

 countries and races. Twins occur among American whites once in every 

 88 births, triplets have a frequency of i:(88)^ and quadruplets appear in 

 the ratio of i:(88)^ Six appears to be the maximum number of simul- 

 taneous births that is well authenticated for man. 



At the outset a distinction must be drawn between true twins and a 

 false type that masquerades under the same name. The coincident produc- 

 tion of two or more individuals is most commonly due to the independent 

 ripening and development of an equal number of independent eggs. In 

 these instances ordinary, or fraternal twins are said to be produced. These 

 offspring may be all of the same sex, or mixed as to sex, and such false 

 twins have only the same degree of family resemblance as occurs in 

 brothers or sisters of different ages. Properly speaking, they are not twins 

 at all but, as in lower animals, merely members of a litter. Quite different 

 are the identical, or duplicate twins. This group includes those true twins 

 characterized by always being of the same sex and so strikingly similar 

 in physical, mental and pathological traits that only rarely is their diagnosis 

 difficult. This identity is enforced by their derivation from a single egg, 

 whereby each member acquires precisely the same chromosomal heritage 

 and hence the same genetic constitution. 



Very rarely identical twins are conjoined as a 'double monster.' All 

 grades of union are known but conjoined twins of equal size have the 

 best chance for survival. 



EMBRYOLOGY AND GENETICS * 



THOMAS HUNT MORGAN 



The stratified rocks of the earth's surface reveal the most recent part of 

 the long history of the evolution of the animals and plants living at the 



* Reprinted from Thomas Hunt Morgan, Embryology and Genetics. Copyright 

 1934 by Columbia University Press. 



