ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 201 



sexual cells. Here, similarly, two cases can be distinguished : (1) where 

 the sexual cells of the hybrid show each only one character, hut both 

 occur, and in equal numbers, i.e. the characters obey Mendel's law; 

 (2) where both characters occur together in the sexual cells. Pairs of 

 characters which behave in the first way Correns calls schizoogonous 

 (isogonous of De Vries), and those of the second class homoogonous 

 (anisogonous of De Vries). If attention is paid only to extreme cases, 

 the author points out that four possible combinations in the behaviour 

 of any pair of characters can obviously be distinguished : (1) hetero- 

 dynamic and schizoogonous; (2) heterodynamic and homoogonous ; (3) 

 homodynamic and schizoogonous; (4) homodynamic and homoogonous. 

 The first and fourth Correns distinguishes as the Pisum and Hieracium 

 type respectively, for, as Mendel showed, most of the characters of these 

 two forms behave on hybridising in the manner indicated. The third 

 Correns characterises as the Zea type, for his observations show that 

 most of the characters of Zea Mays hybrids behave in this special way. 

 The second type is not yet known in nature. It is clearly shown in 

 Zea Mays that some of the characters can behave in one way and some 

 in another ; thus, all the characters fully studied were homodynamic 

 with the exception of the chemical nature of the endosperm, which was 

 found to be heterodynamic. In relation to sexual cell-formation, tbe 

 characters were in part schizoogonous, in part homoogonous. Those 

 taking part in xenia seemed all schizoogonous. The question of the 

 usually preponderating effect in xenia of the characters derived from 

 the female side is to be partly explained by the fact of the fusion of 

 itvo polar (female) nuclei with one generative (male) nucleus. The fact 

 that the sexual cells bearing the heterodynamic characters occur in 

 equal numbers leads Correns to believe that the segregation of the 

 characters is brought about by a nuclear division of the type of Weis- 

 mann's reducing division. The question of the stage of development 

 at which the separation of characters takes place is ably discussed. 

 C>rrens concludes that in the pollen-grain it probably takes place at 

 the time of separation of the vegetative nucleus and generative mother- 

 nucleus ; and in the ovule at the time of formation of the embryo-sac 

 mother-cell. 



Spermatogenesis and Fecundation of Zamia.* — H. J. Webber gives 

 a full account of his investigations on this subject. The pollen-grains 

 are carried by the wind to the female cones, and finally come to lie in 

 the pollen-chamber at the apex of the nucellus. There seem to be three 

 prothallial cells cut off from one side of the grain ; the first is resorbed, 

 as in Gingho and Pinus. In the development of stalk-cell and central or 

 generative-cell Zamia corresponds closely to the Coniferae. The ble- 

 pliaroplasts are formed de novo in the cytoplasm of the central cell ; they 

 are at first very small but gradually increase in size, reaching a maximum 

 at the prophase stage of division of the central cell, when their contents 

 present a beautiful regularly vacuolate structure and stain red with 

 safranin. As the division approaches the equatorial-plate stage the 

 blepharoplasts begin to break up, the contents contracting and gradually 

 disappearing while the outer membrane begins to break apart here and 



* U.S. Dept. of Agricult., Bureau of Plant Industry, Bull. No. 2 (1901) 92 pp. 

 and 7 pis. 



