82 THE AMERICAN ARBACIA 



Jelly. - The mature egg, like the immature, is surrounded by a jelly 

 coat which may be as thick as 32 [x. It has the same refractive index 

 as sea water and cannot be seen with a light microscope, dark field, 

 or phase contrast microscope. It is readily demonstrated with a slight 

 tinge of Janus green or toluidin blue in the sea water. The old method 

 of Boveri (1901) may also be used. He rubbed a stick of India or Chi- 

 nese ink in a drop of sea water to make a thick solution and added the 

 eggs. The jelly is perfectly clear and the India ink particles remain in 

 the surrounding sea water. Also when many sperm are added to eggs 

 in a dish of sea water, many of them are caught in the jelly and form 

 a halo around the eggs. The even spacing of eggs as they are viewed 

 with the microscope is caused by the invisible jelly. If the eggs are 

 contiguous, the jelly is absent and the eggs are in poor condition. The 

 jelly coat may be made to disappear under many experimental con- 

 ditions, e.g., acid, ultraviolet, x-rays, etc. (see under Jelly Layer in 

 Part IV). 



Micropyle. - A funnel shaped micropyle can sometimes be demon- 

 strated in the jelly of both mature and immature eggs by the use of 

 India or Chinese ink or the ink from living squid. I have found the 

 squid ink the most satisfactory, and it is best to use very fresh eggs, not 

 washed. The squid ink makes the jelly swell, often to 60 \l, twice its 

 normal thickness, almost equalhng the diameter of the egg. (See under 

 Jelly Layer, Part IV). Originally observed by Boveri (1901) in Paracen- 

 trotus lividus, the micropyle has been shown in colored drawings in 

 Arbacia eggs, both normal and centrifuged, by Morgan and Spooner 

 (1909) ; they state that it is difficult to detect after the early cleavages. 

 It is much better seen in Psammechinus microtuberculatus and Paracentrotus 

 lividus than in Arbacia, according to Plough (1929). Boveri (1901a, b) 

 and others found that the micropyle marks the place where the polar 

 bodies were given off, the animal pole, and thought it marked the 

 point of attachment of the egg to the ovarian wall. But Jenkinson 

 (191 1 ), Lindahl (1932 b), and Horstadius (1939), for Paracentrotus, and 

 Tennent and Ito (1941) for Mesipilia have found that the polar bodies 

 come off from the free end, and the egg is attached to the ovarian wall 

 at the opposite or vegetal pole. 



The micropyle has been used as a landmark for the localization of 

 materials in the Arbacia and other eggs in polarity studies. Morgan 

 (1909) found that both in the normal and centrifuged egg of Arbacia 

 the micromeres lie approximately opposite the micropyle. (See also 

 Morgan and Spooner, 191 1). Harnley, (1926); Tennent, Taylor, and 

 Whitaker (1929), and Plough (1927, 1929) have also used the micro- 



