Jan., 1906.] Embryo Sac and Embryo of Cucumis sativus. 425 



megaspore begins to enlarge (Figs. 11-12). The latter acquires 

 a very large and distinct nucleus with a nucleolus of unusual 

 size. Kirkwood reported to have found in Trichosanthes that 

 after the division of the megasporocyte the upper cell did not 

 again divide but immediately disorganized, while the lower cell 

 again divided transversely, the upper cell of which also disor- 

 ganized. The ultimate result, however, is the same in both 

 Cucumis sativus and Trichosanthes, that is, the lowest of the 

 megaspores always becomes the functional one. 



The embr^^o sac and its associated structures are quite small 

 in comparison with the very large nucellus. The development 

 proceeds in the normal way, by a longitudinal division of the 

 nticleus of the megaspore (Fig. 13). At this stage often the 

 potential megaspores have not completed their dissolution and 

 remains of the third one can be seen just above the sac. The 

 nuclei arrange themselves at either end of the sac in the center 

 of which is a vacuole across which strands of protoplasm mav 

 extend. In the four-celled stage the large irregular vacuole in 

 the center is also prominent. By two successive divisions the 

 eight-celled embryo sac is formed (Figs. 14-1-5). The svnergids 

 are distinct and lie above the egg. In the early stage thev are 

 somewdiat globular in shape and follow the outline of the sac. 

 They lengthen considerably and at the time of fertilization they 

 are quite long, sac-like structures. The egg is large and extends 

 below the synergids, at first merely protruding a little beyond 

 their base, but before fertilization it becomes much elongated and 

 swollen (Fig. 17). The polar nuclei are unequal in size, the 

 lower one being the larger. They conjugate before the entrance 

 of the pollen tube. No case of double fertilization was observed ; 

 if it occurs it must take place sometime after the polar nuclei are 

 in contact. The antipodals are small cells which lie side by side, 

 in the lower end of the sac. They take the stain more deeply 

 than does the egg apparatus, and for this reason it is often 

 difficult to make out their outline. The}^ do not enlarge but 

 remain in place and are quite distinct even after considerable 

 endosperm has developed. 



The development of the embryo is quite irregular. The first 

 division of the oospore is transverse and the upper cell does not 

 divide further and ma}^ be regarded as a rudimentary suspensor 

 (Fig. 18). At this stage the synergids have begun to dissolve. 

 The second division is by a longitudinal wall, the lower cell alone 

 dividing. Later, one of the lower cells divides by a more or less 

 oblique wall forming a four-celled embryo (Fig. 19) which is 

 almost surrounded by endosperm. Above the embryo the 

 remains of the two synergids can still be seen, although almost 

 dissolved at this time. 



