38 



DEVELOPMENT OF THE FROG'S EGG [Cir. IV 



larger, and two are intermediate in size. The smallest blasto- 

 meres of the upper four always lie nearest the summit of the white 

 crescent; the largest is its vis-d-vis. If we think of the third 

 planes of cleavage as lying in a single plane not quite at right 

 angles to the first and second, but tilted a little, we get a clearer 

 conception of the conditions present. The fourth cleavage- 

 period comes in from a half to three-quarters of an hour later. 

 As an idealized form, we may think of the new planes as form- 

 ing two great circles at right angles to. each other, and lying 

 vertically and between the planes of the first and second cleav- 



B 



Fig. 14.— A. Diagram of four-cell stage to show cross-line. B, C. Sixteen-cell stage 

 of two eggs. (After Rauber.) 



ages (Fig. 13, C). This regular form is rarely if ever attained* 

 and the greatest amount of variation is found to exist. 



Remak said that frog's eggs divide much more regularly 

 when carried in from places where they were normally laid. 

 If allowed to stand quietly after being laid, they soon begin to 

 divide irregularly. Vogt has also observed that those eggs of 

 the salmon develop most regularly that have been kept in 

 motion. It does not seem probable, however, that the motion 

 itself could have anything to do directly with the matter ; but if 

 the egg be not supplied with a sufficient amount of fresh water, 



