MUSEUM OF COMPARATIVE ZOOLOGY. 125 



A furrow appears at one pole. This furrow penetrates into the yolk, 

 forming in profile a slit-like structure, which in this way divides tlie 

 yolk into the 2-cell stage. In Strongylocentrotus, figured by A. Agassiz, 

 we notice that a flattening of each cell of the 4-cell stage occurs prepara- 

 tory to the passage into the 8-cell stage. This flattening occurs on one 

 side at first (p. 710, fig. 27). Several eggs of Echinarachnius, PI. III. 

 fig. 6, were taken in a similar condition. In many others, however, 

 each of the four cells of the 4-cell stage is divided from the very first by 

 a constriction reaching wholly around the cell, PI. II. figs. 4, 8. 



In several ova the following modification of development was observed 

 after the 4-cell stage. Au egg was found in the 4-cell stage apparently 

 normally formed. Pi. II. fig. 9. Immediately after two of the spheres 

 begin to fuse, and the wall of the cleavage plane separating them is 

 broken down. In this way we pass by retrogression from an egg with 

 four, PL III. fig. 9, into one with three segment spheres, PI. III. fig. 12. 

 Whether the many eggs in a 3-cell stage which were observed were all 

 formed in this manner or not, cannot be stated. It was not observed 

 how the 4-cell stage in this abnormal mode of development is formed 

 from the 2-cell stage. Segmented ova with three segmentation spheres 

 are quite common in some trials for artificial fecundation. 



An egg fertilized at noon was found in the 2-cell stage at 1.30 p. m., 

 and passed into the 4-cell stage at 2 p. m. At 3 p. m. it was in the 

 8-cell stage. We can, therefore, roughly say that the formation of a 

 fresh cleavage plane occupies approximately an hour's time. By a 

 comparison with the rate of growth of the stai'fish it will be seen that 

 the rate of development of Echinarachnius is more rapid. The water 

 in which my eggs were kept was evidently warmer than that in which 

 Strongylocentrotus was reared. 



The mode of formation of the 8-cell stage from the 4-cell does not 

 differ from that of the 4-cell from the 2-cell, The segments of the 

 4-cell stage are, however, not always bisected, and here appears the 

 first indication of an unequal segmentation. The spheres of the egg 

 even in the 8-cell stage have a peripheral tendency. In the 8-cell stage 

 it will be noticed, PI. II. fig. 11, that the eight spheres cannot be so 

 brought together as to touch each other on adjacent sides. A recess, 

 cav, is thus early left, which later forms in the interior of the ovum a 

 " segmentation cavity." This cavity increases in size as the size of the 

 segmentation spheres diminishes in the progress of segmentation. An 

 egg in the 32-cell stage was found four hours after impregnation, PI. II. 

 fig. 12. 



