238 haeckel's anthropogenie. 



wliich. have been fused togetlier, totally losing their individuality. 

 But not only cells ; organs also, and even persons, may thus become 

 coalesced with each other. 



We now know that development may take place without such con- 

 crescence (parthenogenesis) ; but usually the two elements, sperm- 

 cell as well as germ-cell, are necessary. These are, as a rule, very 

 different in form and size, the sperm-cell being ordinarily flagellate 

 (rarely amoeboid) and very much smaller than the germ-cell. 



After impregnation the first processes of development are essen- 

 tially the same in all the animal kingdom, and with regard to the 

 mammalian series precisely the same throughout. First, the nucleus 

 disappears, the cell becomes a cytode (monerula — ^it is no longer 

 amcBboid, but moneroid) ; second, a nucleus is formed anew only to 

 initiate the process of fission, which being repeated, results in the 

 formation of a mulberry-like mass of cells (morula). By collection 

 of fluid in the interior of the morula, a vesicle, the blastosphere, is 

 formed, the wall of which consists of a single layer of cells except in 

 one spot (area germinativa), where a little heap of cells remain, and 

 which spot alone is concerned in the formation of the body of the 

 animal. By the growth of the edge of the little heap the blastoderm 

 becomes two-layered, and these layers, exhibiting different chemical 

 and physical characters, are distinguished — the outer as the animal, 

 the inner as the vegetative layer, and correspond to the exoderm and 

 entoderm of all animals except protozoa. 



Such is a summary of the early developmental processes in mam- 

 malia, and essentially the same stages are passed through by all other 

 animals, obscured freC[uently, however, by the presence of food-yolk, 

 disposed in one way or another in the egg. In the bird's egg, e.g., 

 wliich belongs to the discoblastic type of development, change 

 merely takes place in the superficial part of the germinal yolk, and 

 the blastoderm thus formed as a patch on the surface of the egg grows 

 round by its edges, so as to be transformed into a vesicle including 

 the food-yolk. 



In the lower animals there is frequently formed a gastrula stage 

 by the invagination of part of the blastodermic vesicle, the result 

 being an elliptical body with a primitive intestine and a primitive 

 mouth, the wall of the body being formed of the two germinal layers. 



These two layers of the gastrula are homologous with the two 

 layers of the blastoderm ; because from the outer layer all the animal 



