DEVELOPMENT OF SPELERPES BILINEATUS 179 



Cleavage 



Since the external aspect of cleavage differs only in minor 

 details from the usual urodelian form of total type, a very brief 

 account will suffice. 



Regular. It begins from twelve to twenty-four hours after 

 the egg is deposited, three to five eggs of a clutch beginning at 

 about the same time, while the rest follow in similar groups 

 throughout a day. 



The first indication of cleavage is a slight flattening at the upper 

 pole, which soon becomes a deep broad furrow (fig. 1, plate 1). 

 It soon closes, however, except at its ends (fig. 2, plate 1). In 

 only one set was any thing similar to the "Faltenkranzen" of the 

 frog seen. These were very small furrows, varying from one to 

 eight, extending a short distance in various directions from the 

 main furrow. 



Some three hours after the first, the second furrow starts in 

 like manner. The former becomes more distinct, due to a sepa- 

 ration of its edges. This opening and closing of furrows may 

 be observed throughout cleavage. The blastomeres soon shift, 

 giving rise to the usual cross furrow (fig. 4, plate 1). Fig. 5, 

 plate 1, shows one type of the third set of furrows. 



The furrows of the fourth set, as a rule, are the last for a long 

 time to extend below the equator. They do not even reach the 

 lower pole, but usually join the earlier furrows less than 45° 

 below the equator. Thus cleavage is confined to the upper 

 hemisphere of the egg quite like that of Desmognathus fusca, 

 (Hilton, '07). Plate 1, fig. 6, shows the upper hemisphere of an 

 egg after the fifth set of furrows had formed. A later stage is 

 shown in fig. 3, a, b, c, upper, lower, and lateral views, respectively.* 

 There are 50 blastomeres visible from above and only 8 from 

 below. In fig. 4, 130 cells are visible from above and only 7 

 from beneath. Another egg has 400 and 14, respectively. After 

 the time represented by the last, there is a constant increase in 

 the number of lower blastomeres. However, a few eggs were 



* The cavities represented between the cells arc somewhat exaggerated, due to 

 Merkel's fluid. 



