GENERAL EMBRYOLOGY. 197 



budding, the other sexually. The former is called the 

 nurse, the latter the sexual animal. 



17. If more asexual generations follow one another 

 before a sexual animal appears again, one speaks of grand- 

 nurse, nurse, sexual animal. 



1 8. The alternation of parthenogenesis or predogenesis 

 with pronounced sexual reproduction is called regressive 

 alternation of generations or heterogony. 



19. Development which is inaugurated by sexual re- 

 production shows in nearly all multicellular animals a gen- 

 eral agreement in the incipient stages: fertilization, cleav- 

 age, formation of germ-layers. 



20. The essential point of fertilization lies in the com- 

 plete fusion of egg and spermatozoon, particularly in the 

 fusion of the nuclei, egg and sperm nuclei, to form the 

 cleavage nucleus. 



21. T/ic cleavage of tlic egg is a cell division, a division 

 of the fertilized egg into the cleavage spheres. The cleav- 

 age may be total (holoblastic egg) or partial (meroblastic 

 egg); total cleavage is either equal or unequal, the partial 

 either discoidal or superficial. 



22. By progressive division of the cleavage spheres, 

 and by the formation of a cleavage cavity, there arises a 

 one-layered embryo, the blastula (vesicula b)astodermica). 



23. By the invagination of the blastula the gastrula 

 or two-layered embryo arises. 



24. The gastrula surrounds a cavity, the primitive diges- 

 tive tract or archenteron, opening to the exterior through 

 the gastrula-mouth ; it consists of two epithelial layers, the 

 cntoblast (hypoblast) or the inner germ-layer, lining the 

 primitive gut (archenteron), and the ectoblast (epiblast) or 

 outer germ-layer. 



25. Between the inner and the outer germ-layer, still a 

 third, the middle germ-layer, incsoblast, may be formed. 



26. The middle germ-layer arises either by an infold- 

 ing, or a cutting off of a part of the entoblastic epithelium, 

 epithelial mesoblast, mesepithelium ; or by the migration 

 of separate cells to form a gelatinous tissue : mesenchyme. 



