PEOGS AND TADPOLES. 265 



be used as the changes proceed, in the same manner as the 

 albumen of the hen's egg provides the nourishment for the de- 

 velopment of the chick. We must regard the yolk as a single 

 cell, that is, a unit of living substance, or a unit of life. The 

 frog commences life as a single cell and all organisms, vrhether 

 plants or animals, commence life from this very humble origin ; 

 the lowest persisting in this stage throughout their whole exist- 

 ence, while the highest are gradually constructed by a most 

 wonderful series of changes. Tlie frog's eggs at first are spheri- 

 cal, but if we continue to observe them, in about a week they 

 may be seen to become ovoid, and afterwards to increase in 

 length, until about the tenth day they have a slight appearance 

 of division into head, body, and tail. With a pocket lens these 

 changes could be more closely followed; very soon the blackspeck 

 would be seen to divide vertically into two halves and this division 

 is repeated in a plane perpendicular to the first, so that the 

 original cell has now become four cells. The next division will 

 be a horizontal one, but nearer the upper pole, which, it will be 

 observed, is much darker than the opposite extremity. The 

 segmentation proceeds regularly until sixty -four cells have been 

 formed, and the black dot now resembles a number of black 

 heads fastened together, enclosing a hollow space, the upper cells 

 being smaller and more numerous than the lower ones. The 

 second stage, known as the Blastula-stage is now reached, namely 

 a number of cells congregated together at the circumference of 

 a sphere. . 



!N'ow, there are many organisms which advance to this point 

 but never proceed any further ; such for instance as the Volvox 

 glohator, a plant which is familiar to every microscopist. These 

 are merely cell-colonies in which all the cells are exactly similar 

 and equal in all respects. The blastula-stage is found in nearly 

 all, if not all, animals, at an early period of their development. 



The next period is commenced by the cells at the upper end 

 (which we remember were smaller and darker than those at the 

 opposite extremity) gradually increasing and surrounding the 

 larger ones with the exception of one small spot, until the yolk 

 cells are nearly completely enveloped. 



