EMBRYOLOGY. 



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floating on the surface, but in sufficiently early stages to connect them with 

 the stages raised by artificial fecundation. 



In the common sea-urchin of our New England coast (S. Drobaehiensis) 

 the genital organs become mature during the winter, February usually being 

 the month during which artificial fecundation has ordinarily succeeded. 

 The eggs and spermaries at that time fill the internal cavity to such an ex- 

 tent that the alimentary canal is completely surrounded by the clusters of 

 the genital organs. At the time of spawning the eggs are closely packed, 

 pressed into all sorts of shapes, but when they escape in the water, and are 

 allowed to remain in it a short time, the outer envelope, which appears to 

 consist of a thick homogeneous, structureless shell, swells very materially 

 when the pressure is removed, and becomes perfectly spherical {Fig. in). 

 The spermatic particles, as in the case of the artificial fecundation of the 

 starfish, soon find their way to the outer envelope of the egg to which they 

 attach themselves, beating about very violently the whole time. The sper- 

 maries are sometimes so crowded that they form a halo round the outer 

 envelope, as in Asteracanthion. I have not been able to see the spermatic 

 particles reach the surface of the yolk. By the constant beating of the in- 

 numerable spermaries the egg is set in rotation. The first change noticed is 



Fig. 19. 



Fig. 20. 



Fig. 21. 



Fig. 22. 



Fig. 23. 



the disappearance of the germinative vesicle {Fig. 20) ; soon after this the 

 germinative dot vanishes. The first trace of segmentation consists of a sepa- 

 ration of the yolk from the inner wall of the outer envelope {Fig. 2l) ; the 

 yolk is then slightly depressed on one side (Fig. 22), a similar change soon 

 occurring on the opposite pole {Fig. :>,:). 



Segmentation takes place very rapidly, the egg passing in a few hours 

 from the stage of Fig. m to that of Fig. so immediately before the escape of 

 the embryo from the egg. The spheres are well separated, having a centrif- 

 ugal tendency, and as they increase in number they arrange themselves in 

 a shell-like envelope (Figs. 2S-J0), which eventually becomes the wall of the 

 embryo. This centrifugal tendency is already apparent when there are not 

 more than eight spheres, and as early as the stage of Fig. 28 ; when there are 



