EMBRYOLOGY OF ECHINODEKMS. 



103 



transparent spot, the germinative vesicle (c) ; rather diffi- 

 cult to detect in the opaque egg of Arbacia, but more dis- 

 tinct in the egg of Strongylocentrotus. 



b. A few minutes after the egg has been placed in the 

 male fluid, its surface will be found to be thickly covered 

 with spermatozoa, which are attached to it by their 

 " heads," while their " tails " continue in motion with such 

 activity that they may cause the egg to spin or roll 

 through the water. At about the same time the germina- 

 tive vesicle ceases to be visible, although the examination 

 of the more transparent eggs of Strongylocentrotus shows 

 that it does not actually disappear, but undergoes impor- 

 tant changes. As these cannot be ob- a : 

 served in our species, however, they will 

 not be described here. 



FIG. 44. Egg of Arbacia punctulata, a few 

 minutes after fertilization. (From a sketch by 

 H. Gannan. ) 



d. Principal axis. e. Furrow indicating the 

 position of the first cleavage plane. FIG. 44. 



Soon after the germinative vesicle becomes invisible, 

 the yolk (Fig. 44) becomes slightly notched at a point e, 

 upon its periphery, and it is therefore 

 no longer spherical, but divisible into 

 symmetrical halves in the plane (d), of 

 Fig. 44. The axis which lies in this 

 plane is now different from any other 



FIG. 45. Egg at the end of the first period of 

 active segmentation. (From a sketch by H. Gar- 

 man.) 



/. Direction cell. d. Principal axis. 



which can be drawn through the centre of the egg, and 

 is known as the principal axis. In a few minutes more, 



