314 EMBRYONIC DEVELOPMENT 



to believe that there exists a temporary cellular membrane, forming and disappearing 

 alternatively during the labor of the division of the substance. For, the vitelline mem- 

 brane in an egg in process of formation, is itself not more conspicuous, still the yolk's 

 sphere is perfectly circumscribed, though not yet surrounded by either an albumen 

 or a chorion. 



VI. THE GERMINATIVE VESICLE AND GERMINATIVE SPOT. 



The process of the division of the yolk is no sooner started than the germinative 

 vesicle and the germinative spot disappear as vesicles; their contents, then, mix with 

 the yolk. But whether this mixture has any influence upon the process of the divi- 

 sion is difficult to determine. For a long time embryologists thought that the division 

 of the yolk was dependent upon the previous bursting of these vesicles, butone instance 

 is known now, ia which the germinative vesicle is still present, when the yolk is already 

 divided into several spheres. I cannot help thinking that the germinative vesicle 

 and the germinative spot or spots (for there are sometimes several germinative spots 

 in other types), have no initial power towards the future stage of development, and 

 that their existence during the first period of the egg's history, only reminds us of 

 the origin of the eggs in the common laboratory of organic substances. 



The contained matter of the germinative vesicle is transparent; its structure was 

 beyond the reach of the microscopic powers at my command. 



VII. THE CLEAR SPACE IN THE SPHERES OF DIVISION. 



Another phenomenon connected with the division of the yolk, be the division regular 

 or irregular, takes place during this period of the egg's history. 



The vitelline sphere is no sooner divided into two parts, than in fhe centre of each 

 of these parts, a clear space appears, and as the number of the spheres of division in- 

 creases up to the mulberry shape, in the manner above stated, each of the spheres of 

 division exhibits that clear space, however small those spheres may be. 



In its general appearance, this clear space reminds us of tiie germinative vesicle, 

 from which it differs, however, in not being circumscribed by a defined membrane, 

 whence its vague outline, and also in being proportionally larger. That it has nothing 

 in common with the germinative vesicle, is satisfactorily shown in the case where the 

 latter still exists when the yolk presents several spheres of division, each of which 

 being provided with its own clear space. 



The clear space of the spheres of division, therefore, is a phenomenon which indicates 

 in the egg something else than matter, a vital activity every where present in the 

 yolk, endowed with the same power common to all organized beings, to act from the 



