FERTILIZATION AND CLEAVAGE lOI 



with the third edition 1925, p. 395). Recently J. C. Dan (1950) has 

 stated that in six species of Japanese sea urchins and starfish, the tail 

 enters the egg, though none of her figures show it actually inside the 

 egg, only in the perivitelline space, as frequently seen in Arbacia. The 

 question does not seem to be of much importance since the tail has 

 completed its function and is probably of no further use. 



The first cleavage plane has been observed to cut through without 

 any relation to the entrance point of the sperm, not necessarily through 

 it. This has been found to be the case also in Paracentrotus lividus by 

 Horstadius (1928, 1939) who used the Vogt (1925) vital staining 

 technique to locate the sperm entry. However, Wilson (Wilson and 

 Mathews, 1895, P-" 324) says that in Toxopneustes (Lytechinus) "the 

 plane of first cleavage is in the great majority of cases at least approxi- 

 mately through the entrance point of the sperm." See also Wilson's 

 Cell, 1925, p. 1 104. 



Though one sperm, usually enters the egg, the question has been 

 raised whether fertilization is possible with only one sperm. Kite (191 2) 

 injected with a micropipette several sperm into the jelly of an Arbacia 

 egg and obtained fertilization from a single sperm. Glaser (191 5), how- 

 ever, held that more than one sperm is required to bring about changes 

 in the membrane necessary for one sperm to enter the egg. Tyler 

 (1949 c) likewise considers more than one sperm necessary. 



f. The Centriole 



There is no definite single granule or centriole observable in the center 

 of the aster in Arbacia punctulata, either in the sperm aster or the cleavage 

 aster, but a group of small granules. This was stated many years ago 

 by Mathews (Wilson and Mathews, 1895), and was believed to be 

 true also for artificial astrospheres (cytasters) by Morgan (1899). See 

 the discussion by Morgan (1899) and Wilson (1901a). A very skep- 

 tical view concerning the centriole in Echinoderm eggs, especially in 

 Echinarachnius parma, has been held by Fry (1929 and many other 

 papers). See also Wilson's Cell, 1925, p. 677. 



g. Mid-Bodies 



Mid-bodies (Zwischenkorper), thickenings of the disappearing spindle 

 fibers at the equator, are characteristic of plant cells and are very pro- 

 minent in many animal cells, e.g., the spermatogonia! cells of the 

 Orthopteran, Rhomalium (see Fig. 60, p. 138 in Wilson's Cell 1925). 

 These mid-bodies are not conspicuous in Arbacia, though Fry (1937) 



