This view agrees with the conditions found in 

 oyster eggs which have no jelly coat. 



According to the modern view discussed in the 

 re%new of the problem by Runnstrom, Hagstrom, 

 and Perlmann (1959), the jelly coat not only fails 

 to improve fertilization but impedes it by acting 

 as a sieve. Its action may be considered as an 

 elimination process by which the number of 

 spermatozoa capable of attaching to the cytoplas- 

 mic surface is substantially reduced. 



Fertilizin of sea urchin eggs has two distinct 

 properties: it agglutinates sperm suspension and 

 activates the motility of free, single spermatozoa. 

 Both of these properties are present in the fertilizin 

 of an oyster egg. 



ACROSOMAL REACTION 



Spermatozoa of various invertebrates have been 

 found to carry a substance of protein character, 

 probably a lysine, capable of dissolving the 

 vitellme membrane of the egg. Such lysine is 

 present in the sperm of the giant keyhole limpet, 

 Megafhura crenulata (Tyler, 1939), in the sperm of 

 Mytilus, where it is probably located in the 

 acrosome (Berg, 1950; Wada, Collier, and Dan, 

 1956), and in other marine animals (Tyler, 1948, 

 1949). 



Upon contact with the surface of an egg, the 

 spermatozoon undergoes a so-called acrosomal 

 reaction, which is described as the deterioration 

 of the surface of the acrosomal region of the head 

 followed bj' a projection of a stalklike filament. 

 The acrosomal reaction and the discharge of the 

 filament have been observed in starfish, holo- 

 thurians, mollusks, and annelids. The reaction 

 was studied by Col win, A. L., and L. H. Col win 

 (1955) and Colwin, L. H., and A. L. Colwin (1956) 

 in the annelid Hydroides hexagonus and enterop- 

 neust Saccoglossus kowalewskii. Using electron 

 microscopy, the Colwins revealed many interesting 

 details of the penetration of the spermatozoon 

 into egg cytoplasm. In pelecypod mollusks the 

 discharge of the acrosomal filament was observed 

 in Mytilus and in the three species of oysters, C. 

 echinata, C. nipjxnm, and C. gigas (Wada, Collier, 

 and Dan, 1956; Dan and Wada, 1955). The 

 reaction can be induced by egg water as well as 

 by the contact of a spermatozoon with the egg 

 surface. The first sign of acrosomal reaction in 

 oyster sperm is the flattening of the anterior 

 surface of the spermatozoon. At the same time 

 the head becomes extremely adhesive, the acro- 



EGG, SPERM, FERTILIZATION, AND CLEAVAGE 



some membrane bursts, and the filament is 

 discharged. The reaction can be observed when 

 a small drop of live sperm suspension is placed on 

 a cover slip, a minute quantity of egg water is 

 added, and the cover slip then inverted on a slide. 

 The preparation is examined with phase contrast 

 oil immersion lens using anisol (Crown oil) of 

 refractive index 1.515 instead of cedar oU. 



The acrosome reaction of C inrginica is similar 

 to that observed by Dan and Wada in three other 

 species of oysters. Under the effect of egg water 

 the head becomes swollen and rounded and the 

 filament is ejected from the acrosome. The dis- 

 charged filament is wider than the tail and is 

 about three to four times longer than the length of the 

 head (fig. 315 ). In my observations only a small 

 number of oyster spermatozoa suspended in egg 

 water discharged acrosomal filaments. 



The exact role of the filament in the fertilization 

 of oyster eggs is still unknown. Investigations 



Microns 



Figure 31.5. — Diagrammatic drawing of acrosomal reaction 

 in the spermatozoon of C. virginica produced by egg 

 water. Onlj' a part of the sperm tail is shown in the 

 drawing. Drawn from Uve preparation. 



341 



