122 MARGARET C. FERGUSON 



— this force manifesting itself only in the presence of the egg- 

 cytoplasm. 



The demonstration of normal parthenogenesis in several 

 plants and of artificial parthenogenesis by Nathanson ('oo) has 

 led to much interesting discussion regarding the nature of the 

 stimulus exerted by the sperm on the tigg. Klebs ('oi) sug- 

 gests that it is merely of the nature of an external shock, and 

 other explanations have been offered ; but, after carefully 

 reviewing the literature of the subject, Zacharias ('oi) con- 

 cludes that we have still to determine the true nature of the 

 stimulus which the sperm exercises upon the Ggg, and in so far 

 as I am aware, none of the more recent studies have thrown 

 any substantial light on this problem. 



Nothing has been observed throughout this study to indicate 

 that the sperm-nuclei of Piniis ever assume the spiral or reni- 

 form shape, suggestive of spermatozoids, which has been 

 described by recent writers ^ for the sperm-nuclei in various 

 Phanerogams, and by Arnoldi ('oi) in Taxodiimi and Sequoia, 

 But the nuclei early become spherical or elliptical in outline, 

 depending on the breadth of the pollen-tube, and remain so dur- 

 ing their entire later history. 



THE PRO-EMBRYO. 



Division oftheTivo Segmentation-miclei . — The two daughter- 

 nuclei remain in the upper part of the ftgg and pass through 

 the same stages in their development as those described in the 

 maturation of the egg-nucleus, except that, as a rule, no nu- 

 cleolus becomes apparent within them. These nuclei have 

 been observed to approximate in size the mature egg-nucleus ; 

 but they usually cease to grow and begin to divide while they 

 are still much smaller than the fully developed nucleus of the 

 oosphere. The steps in the division of these two nuclei in 

 Pinus Sirobtis^ this division has not been carefully studied in 

 the other species, are almost exactly like those of the first divi- 

 vision. The nuclear reticulum is resolved into a beautiful, open 



^Golinski ('93) in certain grasses ; Nawaschin ('9S), Guignard ('99), Sargant 

 ('99) in Lilium; Guignard ('00) in Tiilipa ; Land ("00) in CompositiP ; Merrell 

 ('00) in Silphium ; Strasburger ('00) in Monotropa ; Thomas ('00) in Caltha ; 

 and many others during the past two years. 



