McKcnncy — Observations on Embryo-sacs. 8 1 



Embryology. 



The following descriptions refer principally to the two 

 species of Scilla which differ but little from each other. The 

 ovules of Scilla are anatropous and are arranged in two tiers, 

 in each of the three cells of the superior ovary. 



The ovules make their first appearance as small knob-like 

 outgrowths from the placenta. No differentiation is discern- 

 ible among the cells of these outgrowths. Soon one of the 

 hypodermal cells grows larger than its neighbors, and is further 

 distinguished from them by the rather denser consistency of 

 its protoplasm and the larger size of its nucleus. This cell is 

 the archesporium. Unlike that of the surrounding cells, the 

 cytoplasm of the archesporium is usually free from sap vacuoles 

 of any appreciable size. During this stage in the life of the 

 archesporial cell, two to three large nucleoli are contained in 

 its nucleus. By the time the archesporium becomes three to 

 four times the size of its neighbor cells, the cells near the free 

 end of the young ovule become raised in an annular welt-like 

 fashion. This is the beginning of one of the integumentary 

 coats, the primine (Plate XI, Fig. i p). The young ovule 

 meanwhile continues to grow and bends slightly on its axis 

 away from its fellow. The archesporium keeps pace in its 

 growth with the ovule, its growth, however, being greater in 

 the long than in the short diameter. The nucleus of the 

 archesporium now divides, and by the deposition of a periclinal 

 wall the archesporial cell is divided into two cells (Plate XI, 

 Fig. i). The smaller and outer of these two cells is the 

 primary tapetal cell (Fig. i T). The formation of the primary 

 tapetal cell is soon followed by another division of the arche- 

 sporium into two unequal cells (Fig. 2). The smaller and 

 outer cell remains passive and at a later period disintegrates. 

 The lower and larger remains in an active condition. 



About the time of the second division of the archesporium, 

 the cells just within the primine become raised up to form a 

 second annular elevation (Fig. 2 s). This is the sccundine 



