CHAPTER 13 



Fertilization and Cleavage 



a. Fertilization. Plates III and V 



The normal development of the Arbacia punctulata egg, from fertilization 

 through the cleavages to the early pluteus of four days, is shown in a 

 series of photographs of the living eggs and of stained sections (Plates 

 III, IV, and V). This was done many years ago ior Lytechinus (Toxop- 

 neustes) variegatus in the classic ''Atlas of Fertilization''' by E. B. Wilson 

 (1895), and the two species are quite similar. For P. lividus see Boveri 

 (1895) and Horstadius (1935). 



When a spermatozoon touches the surface of a mature Arbacia egg, 

 after penetrating the jelly coat, it is engulfed, and a fertilization cone 

 or entrance cone is formed within 20 seconds. The fertilization mem- 

 brane rises over this and quickly spreads around the egg; this takes 

 only about five seconds. The fertilization membrane lifts off, leaving 

 a perivitelline space, at first very narrow, but gradually becoming 

 wider so as to be 3-5 [jt. when fully formed. At first it is usually not 

 equidistant from the surface of the egg, but later becomes so. The whole 

 process from the time that the sperm touches the egg until the fertili- 

 zation membrane is fully raised takes about two minutes at 23 °C. 

 The fertilization membrane is now generally believed to form from the 

 vitelline membrane with the addition of the cortical granule material. 

 See Part IV under Fertilization Membrane, Vitelline Membrane, Cortical 

 Layer. 



The fertilization cone which is raised at the point of entry of the 

 sperm is at first conical or flattened, but later becomes flame-like 

 (Plate III, Photograph 3). It may disappear quickly but often lasts 

 4 to 5 minutes, especially in eggs early and late in the season, and in 

 eggs kept cold. 



In sectioned and stained material, Mathews (Wilson and Mathews, 

 1895) has described the rotation of the sperm head soon after its en- 

 trance into the egg. It rotates through an angle of 180° so that the 

 base containing the sperm aster becomes directed inwards, toward the 

 ^gg nucleus. (See Wilson's The Cell, 3 ed., p. 397). 



