6 EMBRYOLOGY OF THE STARFISH. Part I. 



in which the segmentation is carried on until the whole yolk consists of such 

 minute spheres that it is impossible at first sight to distinguish an egg of a Cteno- 

 phorous Medusa, which has undergone complete segmentation, from one in which 

 the segmentation has not even begun, after the germinative vesicle and dot have 

 disappeared. The disappearance of the germinative dot is accompanied by a sepa- 

 ration of the yolk from the inner wall of the outer envelope of the egg (PL I. 

 Fig. 3) ; this is the first step towards segmentation, and the presence of such a 

 marked interval would greatly facilitate the detection of spermatic particles upon 

 the surface of the yolk, if any of them had penetrated through the outer mem- 

 brane. The first trace of segmentation 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. Fig. 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 segmenta- 

 tion 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 becom.es 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 com- 

 mences before this (PI. I. Fig. 6), and is entirely independent of the motion given to 

 the whole egg by the spermatic particles ; this stops soon after the rotation of the 

 spheres of segmentation has commenced. 



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

 tion 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 Eichtung's-Blaschen of Schultze, which he first noticed in the segmentation 

 of Mollusks, and which were afterwards seen by Lacaze-Duthiers and by Ptobin, 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. I. 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. 1. Fig. 7), retaining always throughout the whole 

 process of segmentation the same relative position to the axis of segmentation (PL I. 

 Figs. 9-17). What part they play in the subsequent history of the embryo, I have 

 not been able to ascertain. Without doubt they hold always the same relation to 



