736 THE POPULAR SCIENCE MONTHLY. 



cellular mass which serves as the first nutriment for the embryo in 

 the seeds of most Phanerogams. The formation of the endosperm 

 has been carefully studied by Strasburger in the embyro-sac of the 

 kidney-bean, and may serve as an example of the process of free-cell 

 formation. The embryo-sac is morphologically a large cell with its 

 protoplasm, nucleus, and cellulose wall, while the endosperm which 

 arises within it is composed of a multitude of minute cells united into 

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

 tion 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 

 appearance. Around each of these as a center 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, consti- 

 tute the completed endosperm. 



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

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

 protoplasm 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 CEdogonium, 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 becomes clear, and a pencil of vibratile cilia here shows itself. 

 The cellulose wall which had hitherto confined it now becomes rup- 

 tured, 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 develops 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, con- 

 sisting essentially of a globule of protoplasm enveloping a nucleus 

 (the "germinal vesicle"), and with one or more nucleoli (the "germi- 

 nal spots ") in the interior of the nucleus. This cell, distinguishable 

 by no tangible characters from thousands of other cells, is neverthe- 

 less destined to run through a definite series of developmental changes, 



