38 



EMBRYOLOGY. 



the beginning of the oviduct ; likewise in the case of Birds they meet 

 in the first of the four regions previously (p. 17) distinguished, and 

 at a time when the yolk is not yet surrounded with its albuminous 

 envelope and calcareous shell. 



In contrast to internal fertilisation stands external fertilisation, 

 which is the simpler and more primitive method, and which occurs in 

 the case of many Invertebrates that live in the water, as well as 

 ordinarily in Fishes and Amphibia. In this method, while male and 

 female keep near together, both kinds of sexual products, which arc 

 for the most part produced in great number, are evacuated directly 

 into the water, where fertilisation takes place outside of the maternal 



tig. 17 A, B, C. Small portions of eggs of Asterias glacialis, after FOL. 



The spermatozoa have already penetrated into the gelatinous envelope which covers the eggs. In 

 A there begins to be raised up a protuberance toward the most advanced spermatozoon. In 

 B the protuberance and spermatozoon have met. In C the spermatozoon has penetrated 

 into the egg. A vitelline membrane, with a crater-like orifice, has now been distinctly 

 formed. 



organism. The whole procedure is therefore much more easily observ- 

 able. The experimenter has it within his power to effect fertilisation 

 artificially, and thus to determine precisely the point of time at which 

 egg and semen are to meet. He needs only to collect in a watch-glass 

 containing water ripe eggs from a female, likewise in a second watch- 

 glass ripe semen from a male, and then to mingle the two in a 

 suitable manner. In this way artificial fertilisation is extensively 

 practised in fish-breeding. For the purpose of scientific investigation 

 the selection of the particular species of animal is of the greatest 

 importance. It is manifest that animals with large opaque eggs do 

 not commend themselves, whereas those species are especially suit- 

 able whose eggs are so small and transparent that one can observe 

 them under the microscope with the highest powers, and at the same 

 time pass in review every least speck. Many species of Echinoderms 



