52 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 
develop because it lacks the apparatus for cell division—the 
centrosome. According to Boveri, the spermatozoon starts 
the development of the egg by introducing into it a centrosome. 
The egg is thus placed in a position to begin its development. 
Only the centrosome can be the cause of division. Now Mead 
points out that in certain eggs, e.g., that of Chaetopterus, 
the spermatozoon has quite a different effect. As long as the 
egg of Chaetopterus is in the ovary, no polar bodies are given 
off; but as soon as it reaches sea-water maturation starts and 
a spindle is formed preliminary to the extrusion of the first 
polar body. 
The egg remains at this stage so long as no spermatozoon 
enters it. 
In the egg of Chaetopterus a perfect amphiaster with centrosomes, 
centrospheres, astral rays, and spindle fibres is developed and the 
egg remains for hours in the metaphase, if it is left unfertilized in sea- 
water; and the same appears to be true of many other marine annelids. 
This elaborate machinery of mitotic division is immediately set in 
motion upon the entrance of the spermatozoon, though the sperm and 
its centrosomes are in a distant portion of the egg. All the phases of 
this and the subsequent mitosis are independent of the karyokinetic 
changes in the vicinity of the sperm. Since in one form the oocyte 
will not divide until the sperm enters the cell, even though the centro- 
somes and the whole amphiaster are present, the suspicion is warranted 
that in the ripe egg of other forms—the sea-urchin, for example—the 
mitosis is not inhibited merely on account of the lack of a centrosome, 
nor is it incited merely because a new centrosome is introduced to 
organize the mitotic figure. 
Mead concludes that for cell division “a stimulus is required, 
analogous, perhaps, to that which starts into activity the 
motor apparatus of pigment cells, leucocytes or muscle cells.” 
In order to pursue this idea farther he made some experiments 
upon the effect of salts, on the advice of C. W. Green. He 
employed the salts of the Ringer solution and found that if 
1A. D. Mead, ‘‘The Rate of Cell Division and the Function of the Centro- 
some,’’ Biological Lectures delivered at Woods Hole, 1896-97, p. 211, Boston, 1898. 
