407 



contained a vesicle destined to disappear. Nothing tended 

 to explain the signification of the egg. 



But in 1839 appeared the book of M. Schwann, and the 

 discovery that all the tissues of animals proceed from cells 

 was destined to effect a radical revolution in science ; the 

 cell-theory of M. Schwann was destined to throw an entirely 

 new light on anatomy and physiology, as well as on em- 

 bryogeny, and suffices to immortalise the name of its 

 author. The cell-theory became established, and the pro- 

 found obscurity which surrounded the question of the signi- 

 fication of the egg and the object of yelk-cleavage disappeared 

 at the same time. Relying on his study of its constitution, 

 M. Schwann was the first to proclaim that the egg is a cell ; 

 and since all the tissues are but a mass of cells, it became 

 obvious that the object of the cleavage was to multiply the 

 cell-egg. Bergmann, Keichert, and Bemak contributed prin- 

 cipally to the demonstration of the j^art which yelk-cleavage 

 plays in the production of cells. 



But although a great number of physiologists — following 

 Schwann — consider the egg as a simple cell, others, Avith 

 Henle, regard the egg as a combination of cells, and see in 

 the germinal vesicle a complete cell. Among these we may 

 cite BischoflT, Steinlein, Stein, «&c. 



It is necessary to take the mean between the two opinions, 

 and to consider certain eggs as simple cells, others as com- 

 plex cells. Can one, for instance, consider as a simple cell 

 the egg of the Trematods, or of the Cestoids, when it is seen 

 to form, by the union in a common shell, of a protojDlasmic 

 cell and of other cells, formed by distinct glands, which have 

 wrongly borne the name ' vitellogenous^ ? " 



The study which Dr. Van Beneden has made of the forma- 

 tion of the egg, and of the first embryonic phenomena, clearly 

 gives the solution of the problem. 



In every egg, whether of Mammifer or of Bird, of Crusta- 

 cean or of Trematod, we find a protoplasmic cell, of which 

 the germinal vesicle is the nucleus, the corpuscle of Wagner 

 (germinal spot), the nucleolus. This cell, which Dr. Van 

 Beneden calls the germ or cell-egg (cellule-oeuf ), and which 

 may be considered as being the first cell of the embryo, 

 arises everywhere throughout the animal series in the same 

 manner ; it presents always the same characters ; and gives 

 origin by division to the first cells of the embryo. 



But the vitellus of the egg is made up of two elements : 

 the one, protoplasmic, rej)resents the mass of the cell-egg ; 

 the other, nutritive, forms what Dr. Van Beneden has desig- 

 nated the deictojilasm of the egg. This deutoplasra is the 



