FERTILISATION 229 



of development, it woiild respond only by forming a double head. 

 Lack of proper oxygen supply also brings about abnormal develop- 

 ment in fish embryos. In Mammals, where we sometimes get 

 polyembryony, it is suggested that an insufficient supply of oxygen 

 to the ovum might produce this condition. The fertilised ovum 

 travelling down the Fallopian tube probably receives an abundant 

 supply of oxygen; arriving in the uterus in certain animals it 

 apparently rests some time before it bedomes fully implanted and 

 the chorionic layer is formed and a free supply of oxygen from the 

 maternal circulation is established, and such a condition might lead 

 to polyembryony. In some animals, as the opossum and the 

 armadillo, polyembryony would seem to be the normal mode of 

 reproduction. In the armadillo, at least, development stops for 

 several weeks after the blastocyst reaches the uterus, and during this 

 time it lies perfectly free in the uterus, without any supply of 

 oxygen from the maternal circulation. 



Newman 1 has brought forward in a recent book many facts 

 of a similar character. 



Artificial Aids to Fertilisation 



It has been already recorded that cross-fertilisation between 

 certain species of Echinoderms can be effected by having recourse 

 to physico-chemical methods. It is not surprising, therefore, that 

 fertilisation between individuals belonging to the same species can 

 be assisted, or caused to take place more frequently, in the presence 

 of certain substances artificially added. 



Thus, according to Eoux, frogs' eggs can be fertilised more readily 

 by adding saline solution to the water in which they are deposited. 

 Wilson says that in the case of the mollusc Patella, a larger number 

 of eggs can be fertilised if potash solution is added.^ Dungern^ 

 states that the activity of the spermatozoa in the sea-urchin can be 

 increased in the presence of substances extracted from the ova. 

 Similarly it is said that normal prostatic secretion has an exciting 

 action on mammalian spermatozoa (p. 248). Furthermore, Torelle 

 and Morgan * have shown that the immature spermatozoa of starfish 

 can be stimulated, and fertilisation can be induced, by adding ether 

 and various salt solutions to the sea-water (see also p. 216). 



1 Newman, The Biology of Twins, Ohicago Press, 1917. 



^ For further information on this subject, with references to literature, see 

 Przibram, Emlyryogeny, English Translation, Cambridge, 1908 ; and .Tenkinson, 

 Experimental IMiryology, Oxford, 1909. 



3 Dungern, "Neue Versuche zur Physiologic der Befruchtnng," Zeitsrh. f. 

 allgem. Phys., vol. i., 1902. 



* Morgan, Experimental Zoology, New York, 1907. 



