184 THE PHYSIOLOGY OF REPRODUCTION 



the growth period of the ovum in the ovary. In Histribodella, 1 

 Dino/i/ii'ltix? S(i<rwirrnx, and a number of Turbellarians, the sperms 

 become attached and enter the egg-cells as soon as these are 

 differentiated in the ovary tissue, but actually fuse with the egg 

 pronuclei only when the egg has completed its full growth and 

 has undergone its maturation divisions. This condition has been 

 especially studied in Setecocimu by Buchneiv' 



It is supposed that the entrance of supernumerary sperms is 

 prevented normally either by some mechanical means, such as the 

 development of a membrane formed after the penetration of the first 

 sperm, or else by a change in the chemical constitution of the ovum 

 occurring as the immediate result of fertilisation. 4 Thus, the brothers 

 Hertwig 5 showed that in the case of eggs the vitality of which had 

 been reduced artificially (e.g. by poisons), the vitelline membrane 

 was formed so slowly after the entrance of the first spermatozoon 

 that others also were able to make their way into the egg cytoplasm. 

 On the other hand, the ova of many animals in which no membrane 

 is formed seem to possess the capacity of resisting the entry of 

 supernumerary spermatozoa, and the same is apparently the case 

 with those ova which have a membrane before fertilisation, this 

 membrane being penetrated by only a single sperm. Loeb c has 

 recently suggested that polyspermy may be prevented by an altera- 

 tion in the surface tension of the egg after the entrance of the 

 spermatozoon. 



McClendon 7 and Gray 8 have shown that there is a rapid drop 

 in the electrical resistance of the ovum 011 fertilisation. Gray finds 

 that in artificial fertilisation brought about by treatment of the 

 eggs with butyric acid, this drop in resistance also takes place. A 

 great drop in the electrical resistance of the egg is brought about 

 by weak trivalent negative salt solutions, such as sodium citrate, 

 while trivalent salts of the positive series such as cerium chloride, 



1 Shearer, " The Anatomy of Histnbodella hoinari" Quar. Jour. Nicr. Science, 

 vol. lv., 1910. 



2 Shearer, " The Problem of Sex Determination in Dinophttus gyrocilatus," 

 ijii-n-. ./",: J//Vr. Science, vol. Ivii., 1912. 



3 Buchner, "Die Besamung der jugendlichen Ovocyte und die Befruchtung 

 bei Saeoodmtlf Arch.f. Zellforsch., vol. xii., 1914. 



4 K,t uner, "On the Structural Constituents of the Nucleus, etc.," Croon ian 

 Lecture, ]'ro>: /{<>>/. .S'w., B., vol. Ixxix., 1907. 



5 Hertwig (O. and B.), " Beitrage zur Kenntniss der Bildung, Befruchtung 

 und Teilung des tierischen Eies," Morph. Jaltr., vols. ii. and iii., 1887. 



1 Loeb (J.), The /tynamin of Liriny M>ift<>r, New York, 1906. 

 ' McClendon, " Electrolytic Experiments showing Increase of Permeability 

 of the Egg to Ions at the Beginning of Development," Science, N.S., vol. xxxii., 



1 J 1 ' ). 



8 Gray (J.), "The Electrical Conductivity of Echinoderm Eggs and its 

 Bearing on the Problems of Fertilisation and Artificial Parthogenesis," Phil. 

 7V./,-.. /;./. >,.. London, Ser. B., vol. ccvii., 1916. 



