CHAXGES IX THE EGG 7 



tion of spermatic particles upon the surface of the yolk, if any of them 

 had penetrated through the outer membrane. The first trace of segmen- 

 tation consists in a depression of the yolk, visible on one side of the 

 sphere (PI. I. Fig. 4), and is soon followed by a similar change on the 

 opposite pole. 



The segmentation takes place very rapidly, passing in about eight 

 hours from the stage represented by PI. I. Fuj. 3 to that of PI. I. Fig. 21, 

 immediately before the escape of the embryo from the egg. The spheres 

 in the earlier stages of segmentation are well separated (PI. I. Figs. 7, 9, 

 11, 13). They have a centrifugal tendency, and, as they increase in 

 number, arrange themselves in a shell-like envelope, which eventually 

 becomes the wall of the embryo. This tendency is already apparent 

 when there are not more than eight spheres (PI. I. Figs. 13, 14) ; and 

 as early as the stage represented on PI. I. Fig. 16, where there are only 

 thirty-two spheres, the envelope is quite prominent. The rotation of the 

 spheres of segmentation commences before this (PI. I. Fig. 6), and is 

 entirely independent of the motion given to the whole egg by the sper- 

 matic particles ; this stops soon after the rotation of the spheres of segmen- 

 tation has commenced. 



As the egg of the Starfish presents nothing peculiar in its process of 

 segmentation beyond what has been just remarked, I refer the reader to 

 the explanation of the plates for the details concerning every successive 

 step of this process, as observed in Asteracanthion berylinus. 



The Richtung's-Blaschen of Schultze, which he first noticed in the seg- 

 mentation of Mollusks, and which were afterwards seen by Lacaze-Duthiers 

 and by Robin, who traced their mode of development, were also observed 

 in the segmentation of the yolk of our Starfish. They are noticed, before 

 the yolk has been divided into halves (PI. 1. Fig. 5), as three or four 

 small granules, situated at the extremity of the axis which is to divide 

 the yolk into two portions (PI. I. Fig. 6). They are developed from the 

 yolk itself as a slight swelling, which afterwards becomes entirely distinct 

 from the mass of the yolk (PI. I. Fig. 7), retaining always throughout 

 the whole process of segmentation the same relative position to the axis 

 of segmentation (PI. I. Figs. 9-17). "What part they play in the subse- 

 quent history of the embryo I have not been able to ascertain. Without 

 doubt they always hold the same relation to the first axis of segmenta- 

 tion, and are, as far as I have observed them in the segmentation of 



