710 



EMBRYOLOGY 



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

 the spheres of segmentation commences before this, and is entirely inde- 

 pendent of the motion given to the whole egg by the spermatic particles ; 

 this stops after the rotation of the spheres of segmentation has commenced. 



The Richtungs Blaschen of Schultze were also observed in the segmenta- 

 tion of the yolk of the sea-urchin. They are noticed, before the yolk has 

 been divided into halves, as three or four small granules situated at the ex- 

 tremity of the axis which is to divide the yolk into two portions (Figs. :/, 24). 

 They are developed from the yolk itself as a slight swelling, which after- 

 wards becomes entirely distinct from the mass of the yolk, retaining always 

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

 of segmentation. The part they play in the subsequent history of the embryo 

 is not known ; they only appear at one pole of the lirst axis of segmentation. 



Fig. 24. 



Fig 25. 



Fig. 26. 



Fig. 27 



Fig. 28 



Fig. 29. 



Fig. :!(). 



Fig. 81. 



Fig. .32. 



Fig. 33. 



Fig. 34. 



Fig. 85. 



After the depression at the poles of the yolk is formed, the segmentation 

 commences 1 Fig. .-;); a slit is formed, which soon divides the whole yolk into 

 two large elliptical masses 1 Fig. 25). These again subdivide into four spheres 

 I Fig. 26) ; in the next stage 1 Fig. 27) the four spheres show the first sign of a 

 further subdivision, and in the subsequent stage (Fig. 28) there are eight 

 spheres. In the stage of Fig. 29 the spheres are small, and show a tendency 

 to form a shell, which is very marked in the next stage figured (Fig. so), 

 where the walls of the embryo are very distinct ; in Fig. SI the segmentation 

 has been carried on till the spheres are very minute and the walls of the 

 embryo most distinct. It is the stage immediately preceding its escape from 

 the egg. In the embryo, when hatched, the walls of one pole at once become 

 thicker (Fig. $2). A depression is formed at that extremity; the embyro at 

 the same time becomes elongated ; this depression is the first trace of 



