1120 THE REPRODUCTIVE ORGANS 



with the chromosomes of the female pronucleus, i.e., with one-half the 

 number allotted to it in sexual reproduction. A similar result is ob- 

 tained during the development of denucleated portions of mature ova 

 when fertilized by spermatozoa. Since the nucleus is the essential 

 factor, the development occurs in the former case without admixture 

 with the male element and, in the latter, without the properties of the 

 female. 



Parthenogenesis may also be incited artificially. Shortly after 

 Bataillon succeeded by means of mechanical impacts in causing unfer- 

 tilized eggs to develop, J. Loeb^ showed that the fertilized egg of the 

 sea-urchin may be prevented from developing by abstracting the oxy- 

 gen from the sea-water by means of KCN or NaCN. In 1899,. this 

 author found that the unfertilized eggs of the same species may be 

 made to develop into larvse by exposing them during a period of two 

 hours to hypertonic salt solutions. ^ By altering the medium by the 

 addition of formic or lactic acid he finally succeeded in causing the 

 unfertilized ova of this and other species to develop their membrane 

 as well as those initial changes which normally require the entrance of the 

 spermatozoon. After their exposure to the aforesaid acids, the eggs were 

 transferred into concentrated sea-water and subsequently into ordinary 

 sea-water. Loeb suggests that the action of the spermatozoon is 

 chemical in its nature, because it brings a certain substance into the 

 ovum which is capable of inciting therein a definite chemico-physical 

 reaction. The nature of this substance is still unknown, although 

 repeated attempts have been made to isolate it. The eggs of the sea- 

 urchin, however, have yielded upon extraction with a hypotonic salt 

 solution and ether a substance which possesses strong fertilizing, 

 agglutinating and cytolytic properties. Furthermore, a chemical 

 substance has been isolated from the head of the Rhine salmon which 

 consists of nucleic acid and a protamin. It is known as salmin. 

 Similar substances are the clupein of the spermatozoa of the herring, 

 and sturin from those of the sturgeon.^ 



The Law of Mendel. — The general conception is that the perplex- 

 ing multiformity among animals and plants is due to the propagation 

 of established forms by heredity, and that new types find their origin 

 in variation. Darwin's ''Origin of the Species" is the first attempt to 

 analyze these phenomena in a rational way and to refer them to natural 

 selection, in accordance with the practice of breeders and experimental 

 botanists to fix characteristics and to produce new ones by interbreed- 

 ing and grafting. In this case, heredity may be amplified by the adap- 

 tation of the individual to dominating conditions, as is clearly depicted 

 by the struggle for existence "and the consequent survival of the 

 fittest. '^ Herbert Spencer, in particular, has made use of this hypoth- 

 esis in explaining many structural and functional characteristics of 



1 Pfliiger's Archiv, Ixii, 895, 249. 



2 Untersuchungen iiber kiinstl. Parthenogenese, Leipzig, 1906. 



3 Burian, Ergebn. der Physiol., i, 1904. 



