INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 821 



from the mother-cell in Gramineje, Elodea, TriglocMn, Carex, Luztila, 

 Tritonia, Sisyrinchiuin, aucl Hemerocallis. 



The walls which separate the daughter-cells, especially that which 

 is formed first, are marked by a strong power of swelling. In all 

 cases the lowest daughter-cell shows considerable increase in size at 

 an early jieriod. It is this only which always develojis into the 

 embryo-sac. Two sacs in a single ovule were once observed in 

 Trujlochin. 



In the dialypetalai, as in monocotyledons, the mode of formation of 

 the cells in the embryo-sac is remarkably constant. The insertion of 

 the two synergidsB in its apex exhibits no variation in any species 

 examined. 



In auatropous ovules the germinal nucleus appears as if fixed to 

 the syuergidic, but is in fact inserted somewhat lower down on the 

 wall of the embryo-sac, and is partly covered by the synergida). The 

 only exception observed was in Hippuris, where the position of the 

 germinal nucleus was always lateral. 



The same regularity recurs in the synergidte always appearing in 

 pairs, and in the presence of a single germinal nucleus ; the only 

 instances in which two germinal nuclei were observed are Gomphrena 

 and Santalum album. 



A reduction in number of the synergidte to one occurs occasionally 

 as an abnormality. The antii)odals are always formed in threes ; in 

 only a single case was a greater number observed, or of their nuclei. 

 They attain a very j)Owerful development in Delphinium and Allionia ; 

 but are transitory and fully developed in Chenopodium, Helianthemum, 

 and Hippuris. 



The central nucleus is always formed by coalescence of the two 

 polar nuclei. Of these either both are motile, meeting in the centre 

 of the embryo-sac, or the upper one remains stationary, and awaits 

 the approach of the lower one. The former occurs in Delpliiuimn, 

 Myosurus, Rihes, Ci/doiiia, Geum, and Ruhus ; the latter in the Centro- 

 sperma), Uelianthemuia, Hippuris, Sanijuisorha, and Agrimouia. 



The mother-cell of the embi'yo-sac always originates, in dialypetala), 

 from the sube2)idcrmal layer, which attains in tliis class its greatest 

 dill'erentiation ; and a resemblance is thus exhibited, through Hippuris, 

 to the behaviour in tlie gamopetalne. Tapeten-cells arc nut always 

 given off; but a uniform behaviour is often to be observed in large 

 circles of aftinity, as in the Centrosperuue. In Chenopodium the 

 division of these cells takes considerable part in the structure of the 

 uucellus. 



Tlie epidermis remains single in a great number of cases ; although 

 a periclinal increase of its elements occurs frequently. The divisions 

 are still more copious -in Delphinium, Helianthemum, and Rosacea), 

 where tliey assist the growth of tlie apex of the nucellus. Hippuris 

 exhibited a peculiar behaviour of the epidermis, resembling that in 

 the ganiopetahe. 



As a rule, only a single embryo-sac mother-cell is formed, as in 

 Centrospcrmi\i, Rannnculacea", and ltibe», though a doubling takes 

 place occasionally. In Helianthemum and Kosaccio several niothcr- 



