FORMATION OF GERM-LAYERS. 



185 



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

 two layers are clearly distinguishable, a flesh-layer and a skin-layer." — Karl 



Ernst Baer (18-28). 



The first processes which occur in the evolution 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 Primaeval Animals, or Protozoa, are peculiar in 

 this respect. These latter include the Monera, Amoeba- , 

 Gregarinae, Flagellata, Rhizopoda, Infusoria, and others. 

 AH these Primaeval 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 oi 

 which fertilization is necessary. Nor do they ever form 

 true germ-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 

 (Echinoderma), Articulated animals (Arthropoda), and Ver- 

 tebrates. 55 



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 



