322 Puget Sound Marine Sta. Pub. Vol. 1, No. 28 



the ovum, and the fluid yolk, it is practically impossible to follow the 

 segmentation in the living egg. Therefore the observations upon the cleav- 

 age and differentiation were made from preserved material. Inasmuch as 

 all the eggs used in the discussion were fertilized at the same time and 

 were kept under identical conditions, the age as well as the stage of de- 

 velopment is used in the description of the segmenting egg. The water 

 in which the eggs were kept varied in temperature from 7^2° to 12° C. 

 during the period of incubation. 



Three hours after fertilization there is no appreciable change in the 

 external appearance of the disk from that of the ripe unfertilized egg (Fig. 

 3). The disk in the 6-hour egg is set distinctly above the granular mate- 

 rial lying on the yolk at the periphery of the disk. This condition re- 

 mains in the segmenting egg until differentiation begins, when the margin 

 of the disk again becomes fused with the granular substance adjacent to 

 it on the yolk. The first cleavage furrow, apparent in the 6-hour egg, 

 starts from the center of the disk and extends transversely across it to 

 either side. This furrow has extended completely across the disk in the 

 8-hour egg (Fig. 5). 



The second cleavage furrow, which is first noticed in the 10-hour 

 egg, also starts from the center of the disk and extends meridionally at 

 right angles to the first furrow (Fig. 6). 



The fourth cleavage is parallel to the second and is complete in all 

 20-hour eggs. The disk at the end of the fourth segmentation is 1 -layered 

 (Fig. 7). 



Twenty-three hours after fertilization the fifth cleavage is effected. 

 This results in a 2-layered mass of cells (Fig. 8). 



The sixth cleavage, at 26 hours, finds the disk still in two layers 

 (Fig. 10). It has not been possible to trace the cleavages following the 

 sixth. 



The first cleavage furrow divides the disk into two blastomeres of 

 equal size (Fig. 5). The four blastomeres resulting from the second cleav- 

 age are not equal in size (Fig. 6). Each succeeding cleavage results in a 

 greater irregularity among the blastomeres until in the 16 and 32 cell 

 disks, none of them is of the same size or shape (Figs. 7, 8, 9). 



There is, however, an apparent bilaterality in the disk during at least 

 the first five cleavages. The disk of the unsegmented egg is circular. The 

 first segmentation produces a disk which is oval in shape, with the longer 

 axis perpendicular to the first cleavage plane (Fig. 5). 



This oval shape of the disk continues to the 26-hour stage when 

 it becomes rounded again (compare Figs. 6, 7, 8, 10). Two circular disks 

 were found among the the 23-hour eggs and one among the 20-hour eggs. 



