244 THEORIES OF EMBRYOLOGY. 



becomes united to the ovule, in the form of a cul de sac, the process of 

 absorption goes on so as to allow the fovilla to reach the embryo-sac. 

 Immediately thereafter, a development of cells takes place in the form 

 of a beaded prolongation or suspensor, at the extremity of which the 

 embryo, in the form of a globular cell, is developed. The free com- 

 munication between the pollen-tube and the embryo-sac ceases after 

 a time ; a constriction takes place by the formation of a diagonal sep- 

 tum, and the pollen-tube either shrivels or continues for some tune 

 adherent to the sac. 



507. Amici also believes that the pollen-tube is applied to the sac 

 of the embryo. He observes in the nucleus a large cavity, which he 

 has called the embryonic vesicle (fig. 435 c). In Cucurbita Pepo and 

 Orchidacese, he traces the tube to a certain depth into the nucleus, 

 and he believes that the granular contents of the tube, which are 

 accumulated at the extremity, are absorbed by the embryonic vesicle, 

 so as to effect impregnation. Cells are then produced hi the vesicle, 



commencing at the base, i.e. opposite to the 

 part where the pollen-tube exerts its influence. 

 The vesicle becomes full of granular matter ; it 

 then exhibits a contraction in the middle, the 

 lower part becoming appropriated to the em- 

 bryo, and forming the true embryo-sac, while 

 the upper part, in such plants as Orchis Mono 

 and mascula, elongates upwards (fig. 435 e), 

 | forming a compound filament, composed of cells 

 with fluid contents. This filament traverses 

 in an inverse manner the course followed by the 

 pollen-tube, and passes into the ulterior of the 

 placenta, being quite distinct from the pollen- 

 435 tube, and probably connected in some way 



with the nutrition of the embryo. Sometimes the pollen-tube re- 

 mains after the embryo has multiplied its cells. Hartig thinks that, in 

 different instances, the mode of impregnation is different. Thus he 

 admits that the true pollen-tube comes into contact with the ovule 

 in Coniferae, that in some Cruciferse the tubes in connection with the 

 ovule are derived from the conducting tissue of the style, as main- 

 tained by Gasparrini, while, in certain Cupuliferse, tubes proceed 

 from some part of the ovule itself. 



508. A fourth theory is, that the pollen-tube (fig. 436 p f), after 

 reaching the ovule, enters the foramen, ex and en, and then penetrates 

 the embryo-sac, es, or pushes the sac before it, becoming thus en- 

 closed in a reflection of it. This view is supported by Schleiden, 



Fig. 435. Ovule of Orchis mascnla, illustrating Amici's view of fertilization, a, Primine. 6, 

 Secundine. c, Embryo. , Confervoid filament .which proceeds from the embryo towards the 

 placenta, and is independent of the pollen-tube. 



