794 



EMBRYOLOGY 



Crossing of pure germ-cells of parent-stocks presenting different 

 characters produces a hybrid in which the characters of one parent only 

 appear, this being the dominant. 



Hybrids, however, although presenting only the dominant char- 

 acters of one parent, may transmit to their offspring, characters of both 

 parents, the recessive as well as the dominant. 



In cases in which a hybrid presents individual characters of its own, 

 differing from those of either parent, it is probable that the distinctive 

 characters are more or less remotely ancestral in their origin. 



The laws of Mendel bid fair to reduce the breeding of animals to 

 an almost exact science. It is evident, however, that breeding in the 

 human subject can never be conducted on scientific lines, however de- 

 sirable this might appear. Still, in studying the fertilization of the 

 ovum as will be seen farther on the idea has occurred to embry- 

 ologists that hereditary transmission is effected through the male and 

 the female chromosomes. It is well known, also, that the offspring of 

 intermarriages often present intensified hereditary characters derived 

 from one parent or the other, as is illustrated in inherited predisposi- 

 tion to certain diseases, such as tuberculosis and cancer. 



The observations of Mendel have received but little attention at the 

 hands of biologists. They are contemporary with what is known as the 

 evolution-theory, which was propounded by Darwin in 1859. Recent 

 discoveries in biology are more favorable to the cytologists, who adopt 

 the cell-theory of inheritance, than to the evolutionists, who knew little 

 of the mechanism of mitosis, although Virchow's aphorism " omnis 

 cellula e cellula " dates from 1855. 



Hereditary Transmission, Superfeciindation etc. The first question 

 that naturally arises relates to the conditions which determine the sex 

 of offspring. Statistics show the proportions between male and female 

 births ; but nothing has ever been done in the way of procreating male 

 or female children at will. According to Longet, the proportion of 

 male to female births is about 104 to 105, these figures presenting cer- 

 tain modifications under varying conditions of climate, season, nutri- 

 tion etc. It has been shown, by observations on certain of the inferior 

 animals, that the preponderance of sex in births bears a certain rela- 

 tion to the vigor and age of the parents; and that old and feeble 

 females fecundated by young and vigorous males produce a greater 

 number of males, and vice versa ; but no exact laws of this kind have 

 been found applicable to the human subject. 



Reference has frequently been made to the chromosomes as the parts 

 concerned in the transmission of inherited characters. This view, which 

 is now at least provisionally adopted by most embryologists, rests 



