FORMATION OF GERM-LAYERS. 1 8$ 



liar organization. In the upper part also (the animal or serous germ-layer) 

 two layers are cleai'ly distinguishable, a flesh-layer and a skin-layer." — Karl 

 Eknst Baer(I828). 



The first processes which occur in the evohition of the 

 individual, after the impregnation of the egg-cell is com- 

 plete, and after the formation of the parent-cell, are essen- 

 tially similar throughout the whole animal kingdom, and 

 always begin with the so-called yelk-cleavage, and the 

 formation of the germ-layers. Only the lowest and simplest 

 animals, the Primseval Animals, or Protozoa, are peculiar in 

 this respect. These latter include the Monera, Amcebse, 

 Gregarinte, Flagellata, Rhizopoda, Infusoria, and others. 

 All these Primseval Animals reproduce themselves, as far as 

 we yet know, only asexually, by division, the formation of 

 buds, spores, germ-cells, and so on. On the other hand, they 

 never have true eggs, i.e. germ-cells, to the evolution of 

 which fertilization is necessary. Nor do they ever form 

 true g3rm-layers. All other animals, on the contrary, all 

 true animals, or Metazoa (as we may call them, in contra- 

 distinction from the Protozoa) have true eggs, and, from their 

 impregnated eggs, form true germ-layers. This is as true 

 of the low Plant-animals and Worms, as of the higher 

 developed Soft-bodied animals {Mollusca) Star-animals 

 (EchinodeTTna), Articulated animals {Arthropoda), and Ver- 

 tebrates.^^ 



The most important processes of germination are essen- 

 tially similar in all these true Animals (the Primaeval animals 

 being excluded). In all, the parent-cell, which arose from 

 the fertilized egg-cell, separates, by repeated cleavage, into 

 a large number of simple cells. All these cells are direct 

 followers or descendants of the parent-cell, and, for reasons 



