GENERAL EMBRYOLOGY. 163 



12. In rare cases the egg develops without fertilization: 

 parthenogenesis; this is a sexual reproduction with degenerated 

 fertilization. 



13. Pcedogenesis is parthenogenetic reproduction by a young 

 (i.e., incompletely developed) animal. 



14. Different modes of reproduction (asexual, sexual, partheno- 

 genesis, paedogenesis) may occur in the same species; then these 

 often occur in a regular order, and in such a way that individuals, 

 with different modes of reproduction alternate' with one another:. 

 alternation of generations in the wider sense. 



15. Alternation of generations in the strict sense (progressive^ 

 generation, metagenesis] is the alternation of two generations, of 

 which one reproduces by division or budding, the other sexually.. 

 The former is called the nurse, the latter the sexual animal. 



16. The alternation of parthenogenesis or paedogenesis with 

 pronounced sexual reproduction is called regressive alternation of 

 generations, or lieterogony. 



17. Development which is inaugurated by sexual reproduction 

 shows in nearly all multicellular animals a general agreement in 

 the incipient stages: fertilization, cleavage, formation of germ- 

 layers. 



18. The essential point of fertilization lies in the complete 

 fusion of egg and spermatozoon, particularly in the fusion of the 

 nuclei, egg and sperm nuclei, to form the cleavage nucleus. 



19. The cleavage of the egg is a cell division, a division of the 

 fertilized egg into the cleavage spheres (blast omeres). 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. 



20. By progressive division of the cleavage spheres, and by the 

 formation of a cleavage cavity, there arises a one-layered embryo, 

 the Uastula (vesicula blast odermica). 



21. By the invagination of the blastula the gastrula or two- 

 layered embryo arises. 



22. The gastrula contains a cavity, the primitive digestive 

 tract or archenteron, opening to the exterior through the blasto- 

 pore ; it consists of two epithelial layers, the entoderm (hypoblast) 

 or the inner germ-layer, lining the archenteron, and the ectoderm 

 (epiblast) or outer germ-layer. 



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

 the middle germ-layer, mesoderm, may be formed. 



24. The middle germ -layer arises either by an infolding or 



