352 EYCLESHYMER. [Vol. X. 



row appearing in quadrant d, passing from the first to the third 

 vertical in a horizontal direction. The next furrow appeared 

 in quadrant a 2 min. later, extended in the direction inter- 

 mediate between horizontal and vertical, and terminated in the 

 second vertical ; i min. later a horizontal passed from the 

 second to the third vertical in quadrant c. In a few moments 

 horizontals appeared simultaneously in the adjoining cells of 

 quadrants b and d. A horizontal i min. later passes between 

 the third and first verticals in quadrant a ; i min. later verticals 

 depart from the first vertical in quadrants b and c. 



Although I have never found this cleavage entirely vertical, 

 it has been observed in the Frog by Prevost and Dumas ('24). 

 They say : " Au meme instant deux nouveaux sillons parall61es 

 a celui qui s'etait montre le second sur la partie brune, viennent 

 se dessiner sur elle d'abord sous la forme d'une trace leg^re, 

 et bientot ils atteignent une profondeur analogue a celle de 

 leurs predecesseurs. Cet hemisphere se trouve alors divis6 

 en seize parties egales ou a peu pres." 



Later cleavage. — The following cleavage (Fig. 10) is like- 

 wise made up of both verticals and horizontals. The majority 

 are generally vertical, occasionally horizontal. The correspond- 

 ing diagram (Fig. 17) illustrates the times of the divisions. 

 The interval from the beginning of the first to the last furrow 

 is 1 5 min. A vertical section through the lighter and darker 

 regions of such an egg is shown in Fig. 20. The cells forming 

 the roof of the segmentation cavity (sc) vary in size and 

 pigmentation, the larger being more deeply pigmented. 



In the next cleavage (Fig. 11), as in the preceding, the 

 furrows pass in both horizontal and vertical planes. In the 

 corresponding diagram (Fig. 18) the times are but partially 

 recorded ; it being at this time extremely difficult to observe 

 the cleavage over the entire surface. The interval between 

 the first and last furrows is 20 min. 



The differences in color observed in earlier stages have 

 gradually become more pronounced, so that at present in nearly 

 all eggs differences exist. The lighter portion fades out gradu- 

 ally in the region of the equator, while the darker portion has 

 a more distinct line of demarcation. In many eggs the surface 



