THE DEVELOPMENT OF LA3IELLIBEANCHS. 



319 



got into such a large vessel that it is almost impossible to 

 find any of them for microscopic examination. 



II. The segmentation and development of the egg. 



The following description has been written from the 

 eggs of the oyster, but it will be found 

 to apply pretty exactly, except as regards 

 time, to the developing eggs of other lamelli- 

 branchs. 



Fig. IGO. — Egg about thirty minutes after fertiliza- 

 tion. 



Fig. 160. 



About fifteen minutes after the eggs are fertilized, they 

 will be found to be covered with male cells, as shown in 

 Fig. 159. In about an hour the egg will be found to have 

 changed its shape and appearance. It is now nearly 

 spherical, as shown in Fig. 160, and the germinative ve- 

 sicle is no longer visible. The male cells may or may not 

 still be visible upon the outer surface. In a short time, a 

 little transparent point makes its appeamnce on the sm- 

 fiice of the egg, and increases in size, and soon forms a 

 little projecting transparent knob, — the polar globule, or 



direction cell, — which is shown in Fig. 



161, and in succeeding figures. 



Fig. 161. — Egg two hours and eighteen min- 

 utes after fertilization ; drawn with the formative 

 pole of the principal axis at the top of the 

 figure. 



a. Maeromere. 6. Anterior micromere. c. Pos- 

 FiG. 161. terior micromere. 



Eecent investigations tend to show that while these 

 [changes are taking place, one of the male cells penetrates 

 [tile protoplasm of the egg, and unites with the germina- 



