MONOGRAPH OF THE EXISTING CRINOIDS. 439 



funnel-shaped opening is seen, at the base of which lies the very narrow (cres- 

 centic) slit representing the blastopore opening of the preceding stage. 



At this stage the cells recently invaginated differ somewhat from those earlier 

 pressed inward, but later an equalization of the differences occurs and a uniform 

 entoderm results, from which mesoderm cells are usually given off as freely in the 

 immediate vicinity of the blastopore as elsewhere, though frequently only from a 

 circumscribed region at the blind end of the primitive gut. 



During the succeeding hours the formation of mesenchyme continues, while at 

 the blastopore invagination still occurs, though now only to a limited extent. 



In these processes numerous cases of individual variation can be observed, not 

 only in embryos of different animals, but also in those of the same pinnule of the 

 same animal. The shape and form of the blastopore especially varies, it being 

 frequently elliptical, and sometimes very small and almost circular. 



The increasing thinness of the ectoderm layer and the decreasing size of its 

 constituent cells is explained by the loss of material through the extensive invagi- 

 nation of the entoderm and the formation of the mesenchyme. 



The bilateral condition of the embryo has now become strikingly disturbed, 

 for it is always to be observed that the left side of the end of the primitive gut lies 

 nearer the ectoderm than the right, and that as a consequence mesenchyme cells are 

 more numerous on the right side of the primitive gut than on the left. 



CLOSURE OF THE BLASTOFOKE. 



In the stage last described (24 to 26 hours) the blastopore was perceptibly nar- 

 rowed. This process continues, and 36 hours after the inception of cell division 

 practically all of the embryos have the blastopore closed and the primitive gut in 

 the form of an isolated sack. 



The embryo is now, as in the preceding stage, approximately spherical; its 

 diameter has become somewhat larger than that of the egg, measuring 0.27 mm. 



In the cases where the blastopore in young embryos exists as a narrow slit 

 the closure takes place in two different ways, beginning either at or between the 

 ends. If the closure begins toward the middle of the blastopore and does not ad- 

 vance with exactly the same speed on both sides, the last trace of the blastopore does 

 not lie precisely in the major axis. 



The line of closure can be demonstrated in the ectoderm for a considerable 

 time as a channel-like shallow depression, which is indicated by the small size of 

 the bordering ectoderm cells and by a slight indentation. This furrow persists 

 for a long time after the detachment of the entoderm. 



When once the adjacent cells have become the same as the other ectoderm cells 

 the furrow vanishes. The smoothing out of this furrow follows the same course 

 as the original closure of the blastopore. At the last only a socket-like cavity can 

 be detected over the place where the blastopore was, and soon even this disappears. 



Meanwhile the primitive gut has assumed the form of a constricted sack and 

 from the presence of the groove over the location of the original blastopore it is 

 possible to determine that this constriction has taken place in the direction of the 

 major axis. 



142140 21 Bull. 82 30 



