GENERAL EMBRYOLOGY. 195 



mation of the blastoderm. In the case of many ivorms the 

 egg-shell may contain, even at the time of laying, an ani- 

 mal all ready for hatching. 



No Sharp Line to be Drawn between Oviparous 

 and Viviparous. Transitional forms of this kind show 

 that between "egg-laying" and "bearing living young" 

 no sharp line can be drawn, and one must guard against 

 attributing too much importance to the apparent distinc- 

 tions. It was entirely erroneous for Linnaeus, following 

 the example of Aristotle, to regard the time of birth as of 

 systematic importance. In many divisions of animals, ovip- 

 arous as well as viviparous forms are found. The major- 

 ity of sharks are viviparous, but a few species lay eggs; on 

 the contrary, for bony fishes the rule holds that the eggs 

 are laid before fertilization. Exceptions to this are the 

 viviparous Zoarccs viviparus and others. Most of the am- 

 phibia, reptiles, and insects are egg-layers, but not a few 

 forms are viviparous. Even among the mammals, for 

 which for a long time the " bearing young alive " was re- 

 garded as diagnostic, there have been discovered lately 

 egg-laying forms, the Echidna and the Ornithorhynchus. 

 Finally, exceptions to the rule occur in one and the same 

 species. Adders commonly lay eggs, but under unfavorable 

 conditions they retain them inside their body until ready 

 to hatch. 



SUMMARY OF THE FACTS OF ONTOGENY. 



1. The development of an animal begins with an act of 

 generation; spontaneous generation and generation by 

 parents are to be distinguished. 



2. Spontaneous generation (Generatio aequivoca, orspon- 

 tanea; Abiogenesis) is the origin of living beings from life- 

 less matter (without pre-existing organisms). 



3. The present existence of spontaneous generation is 

 neither shown by observation, nor is it, on the whole, prob- 

 able; yet spontaneous generation is a logical postulate, in 

 order to explain the first origin of life on our globe. 



4. Generation by parents (Tocogony), derivation of an 



