84 INTRODUCTrON. 



figure from Bcker's ' Icones PhysiologicEe ' will give 

 an idea. 



This type of segmentation, in wliich the first furrow 

 completely cleaves the whole egg into two, each suc- 

 cessive one again subdividing that cell with which 

 it is related, the first two furrows vertical and the 

 third horizontal, is termed complete or holoblastic. 

 The first plane -of division passes through the plane of 

 symmetry of the future embryo, the tissues of which 

 are formed from these cells. The egg of Alytes 

 contains a much greater quantity of nutritive matter, 

 and the segmentation is less regular and more limited ; 

 it belongs to the merohlastic type. 



The time required for the eggs to hatch varies con- 

 siderably, according to the species. Thirty hours 

 may suffice for those of Discoglossus pictus, whilst 

 those of Alytes obstetricans take at least, under the 

 best conditions, nearly three weeks. The eggs of 

 other species fall between the two extremes, the 

 evolution being of course accelerated or retarded by 

 the variable temperature with which early breeders 

 have to contend, or even temporarily arrested when 

 the thermometer descends to freezing-point or below. 



