Embryology. 13 



Up to this point, there is no difference in the principles of the forma- 

 tion of the parent egg amongst all the animals above the protozoans 

 except in the case of certain insects, such as plant-lice, &c. , which 

 produce in a line of females for a number of generations during a sum- 

 mer without the help of the male principle, a process that is called 

 Parthenogenesis or < 'virgin production''*. With these we have nothing 

 to <lo for the present. 



The male and female cells that form the parent cell, as above 

 described, are in many of the animals all carried in one individual, and 

 such animals are called Hermaphrodite. Some of the very simple 

 polypes and sponges, and also many worms, leeches and snails, and the 

 ascidians, &c. , are all hermaphrodite. In the case of the lowest verte- 

 brate, the little fish, amphioxus, or lancelet, the two kinds of eggs are 

 carried by separate individuals, the male carrying the male cells, or 

 spermatozoa, and the female the egg ce ] ls or ova. It is significant that 

 there is no other difference between the male and female lancelet than 

 that of the separation of the two kinds of cells; all the other organs of 

 the two are precisely alike. 



FIG. 19. 



FIG. 19. Egg cleavage and gastrulation of human egg. a. Impregnated parent cell 

 Ci/iti/u. />. First cleavage into a large bright cell, the mother of the ezoderm, and a 

 smaller dark one, mother of the entoderm. c. Cleavage into 4 cells, d. Beginning of 

 the inversion by \vhich the durk cells are unclosed hy the bright ones, e. The process 

 further advanced. /.Hood gastrula formed. (Haeckel.) 



Now to return to the impregnated egg, we will trace its development 

 as it takes place with the mammals, the class of the vertebrate sub- 

 kingdom to which man belongs. 



After the impregnation a new nucleus is formed, as above stated, and 



* This is where one impregnation descends from mother to daughter through several 

 generations before it wears out. 



