THE CELL-DOCTRINE OF PHYSIOLOGY. 135 



or the germinal vesicle of the egg, is the nucleus of the first; 

 and the third, which is called by embryologists the germinal 

 spot, is a nucleolus to the second. It appears from the re- 

 searches of Wagner and Barry, that the nucleus or germinal 

 vesicle precedes the formation of the vitelline membrane, but 

 the precise relation, as to the period of its formation, of the 

 nucleolus or germinal spot to the nucleus, has not yet been 

 satisfactorily made out. The germinal vesicle and spot become 

 the seat of a series of changes, which give rise to the develop- 

 ment of new cells, for the formation of the embryo. 



" At this period the embryo consists of an aggregate of cells, 

 and its further growth takes place by the development of new 

 ones. This may be accomplished in two ways : first, by the 

 development of new cells within the old, through the subdivi- 

 sion of the nucleus into two or more segments, and the forma- 

 tion of a cell around each, which then becomes the nucleus of 

 a new cell, and may in its turn be the parent of other nuclei : 

 and secondly, by the formation of a granular deposit between 

 the cells, in which the development of the new cells take 

 place. The granules cohere to each other in separate groups 

 here and there, to form nuclei, and around each of these a 

 delicate membrane is formed, which is the cell-membrane. 



" In every part of the embryo the formation of nuclei and 

 of cells goes on in one or both of the ways above mentioned ; 

 and by and by, ulterior changes take place, for the production 

 of the elementary parts of the tissues." 



The mode of development just referred to may be illustrated 

 by the following cuts. Fig. 41 exhibits a section of one of 

 the branchial cartilages of the young tadpole. Within the 

 large parent-cells, that are held together by intercellular sub- 

 stance, a, I), Cj we observe secondary cells in various stages of 

 development : at d, the nucleus is single ; at e, it is dividing 

 into two ; in the adjoining cell, the division into two nuclei 

 d f and e', is complete ; at /*, two such nuclei are enclosed within 



