Gametogenesis, Fertilization and Parthenogenesis 



195 



collisions are ineffective owing to such factors 

 as "muzzling" of the spermatozoa by fertili- 

 zin in solution, necessity for particular orien- 

 tation of sperm head to egg surface, etc. 

 While this analysis involves several assump- 

 tions, of which the authors are well aware, 

 it represents an excellent approach to the 

 solution of this perplexing problem. The 

 present author, too, is inclined to favor the 

 view that the block to polyspermy may be 

 established mvich more slowly than is gen- 

 erally assumed. Experiments on the so-called 



enters, in others a hyaline process persists for 

 some time — for example, as long as 15 

 minutes in Urechis (Tyler, '31). Of special 

 interest is the formation (Fig. 63), by star- 

 fish eggs, of a long filament originally de- 

 scribed by Fol (1877, 1879) and more re- 

 cently by Chambers ('30) and Horstadius 

 ('39). The filaments extend from the surface 

 of the egg to spermatozoa in the outer part of 

 the gelatinous coat and their contraction 

 brings the sperm to the surface. According 

 to Horstadius ('39) the filament has the 



^'mmm 



54' i^ ■•',,' '.i'^/.V-^.V 



Fig. 63. Reproduction of some of Fol's ('79) figures of fertilization in Asterias glacialis, showing filament 

 connecting the spermatozoon with the surface of the egg. Three successive stages for one egg are illustrated 

 in a, b, and c, and for another egg in d, e, and /. 



reversal of fertilization in Urechis (Tyler 

 and Schultz, '32), in which treatment of the 

 eggs with weak acid within as much as 

 two or three minutes after insemination 

 stops development and permits subsequent 

 refertilization, accord with this view. 



Fertilization Cones. It is well known that 

 the fertilizing spermatozoon becomes motion- 

 less upon attachment to the surface of the 

 egg. Penetration is, then, not effected by 

 mechanical activity of the sperm. In many 

 species of animals the egg elevates, at the 

 point of sperm contact, a cone-shaped process 

 that engulfs the sperm. Examples of various 

 types of entrance cone formation may be 

 found in studies by Chambers ('30, '33) and 

 these, perhaps, suffice to emphasize that the 

 process differs so markedly in different spe- 

 cies that the common element in each is 

 difficult to discern. While in some species 

 the elevation may disappear as the sperm 



form of a hollow cylinder, and Tyler ('48a) 

 interprets it as a precipitate resulting from 

 fertilizin-antifertilizin interaction. 



Membrane Elevation. While the elevation 

 of a membrane upon fertilization is not 

 characteristic of fertilization in all groups of 

 animals, its occurrence is a manifestation of 

 important surface changes that occur upon 

 fertilization. Recent studies of this process 

 have been largely confined to echinoderms 

 and are reviewed by Runnstrom ('49) who, 

 with his co-workers, has contributed exten- 

 sively to the subject. According to these 

 workers the fertilization membrane of the 

 sea lu-chin egg forms as a result of the cortical 

 granules of the unfertilized egg merging 

 with the vitelline membrane (Fig. 64). Its 

 elevation is attributed to the osmotic pressure 

 of colloids below it. 



Since the fertilization membrane can be 

 removed shortly after fertilization without 



