ADDRESS. 17 



into a tissue. The formation of the endosperm is preceded by the disso- 

 lution and disappearance of the nucleus of the embryo-sac, and then in the 

 midst of the protoplasm of the sac several new nuclei make their appear- 

 ance. Around each of these as a centre the protoplasm of the mother 

 cell is seen to have become differentiated in the form of a clear spherule, 

 and we have thus corresponding to each of the new nuclei a young naked 

 cell, which soon secretes over its surface a membrane of cellulose. The 

 new cells, when once formed, multiply by division, press one on the other, 

 and so combining into a cellular mass, constitute the completed endo- 

 sperm. 



Related to the formation of new cells, whether by division or by free 

 cell-formation, is another very interesting phenomenon of living proto- 

 plasm known as 'rejuvenescence.' In this the whole protoplasm of a 

 cell, by a new arrangement of its parts, assumes a new shape and acquires 

 new properties. It then abandons its cellulose chamber, and enters on a 

 new and independent life in the surrounding medium. 



A good example of this is afforded by the formation of swarm-spores 

 in Oedogonium, one of the fresh- water Algae. Here the whole of the 

 protoplasm of an adult cell contracts, and by the expulsion of its cell- 

 sap changes from a cylindrical to a globular shape. Then one spot be- 

 comes clear, and a pencil of vibratile cilia here shows itself. The cellu- 

 lose wall which had hitherto confined it now becomes ruptured, and the 

 protoplasmic sphere, endowed with new faculties of development and 

 with powers of active locomotion, escapes as a swarm spore, which, after 

 enjoying for a time the free life of an animal, comes to rest, and de- 

 velopes itself into a new plant. 



The beautiful researches which have within the last few years been 

 made by the observers already mentioned, on the division of animal cells, 

 show how close is the agreement between plants and animals in all the 

 leading phenomena of cell-division, and afford one more proof of the 

 essential unity of the two great organic kingdoms. 



There is one form of cell which, in its relation to the organic world, 

 possesses a significance beyond that of every other, namely, the egg. As 

 already stated, the egg is, wherever it occurs, a typical cell, consisting 

 essentially of a globule of protoplasm enveloping a nucleus (the ' germinal 

 vesicle'), and with one or more nucleoli (the 'germinal spots') in the 

 interior of the nucleus. This cell, distinguishable by no tangible cha- 

 racters from thousands of other cells, is nevertheless destined to run 

 through a definite series of developmental changes, which have as their 

 end the building up of an organism like that to which the egg owes its 

 origin. 



It is obvious that such complex organisms as thus result — composed, it 

 may be, of countless millions of cells — can be derived from the simple 

 egg cell only by a process of cell-multiplication. The birth of new cells 

 derived from the primary cell or egg thus lies at the basis of embryonic 

 development. It is here that the phenomena of cell-multiplication in the 

 1879. c 



