8 OUTLINES OF CHORDATE DEVELOPMENT 



region and a narrow peripheral region which is occupied by 

 several rows of small clearer vacuoles. It is doubtful whether 

 there is a true cellular egg follicle: a thin layer of cells forming 

 the wall of the primary ovarian cavity forms an ovarian follicle 

 which folds in and covers almost completely the superficial 

 oogonia. 



After the growth period is completed the process of maturation 

 commences while the oogonia are still within the ovary. The 

 chromatic nuclear membrane disappears and the whole nuclear 

 apparatus moves toward the atrial (i.e., attached) surface of 

 the cell, marking the animal pole and establishing visibly the 

 symmetry of the egg. As it reaches the surface a typical polar 

 spindle with asters forms, and the chromatin forms into twelve 

 tetrads, indicating 'a somatic chromosome number of twenty- 

 four. About this time the very thin vitelline membrane is 

 formed on the surface from the substance of the egg proper 

 (Fig. 1), and as the first polar body is cut off it pushes this before 

 it, so that a portion of the membrane is cut off with the polar 

 body which may no longer remain attached to the egg. Imme- 

 diately after the first polar body is given off the second polar 

 spindle forms, but the division halts in the mesophase (Fig. 1). 

 Just before or during the formation of the first polar body the 

 inner wall of the secondary ovarian cavity is ruptured by con- 

 traction of the body wall and of muscle fibers in the thin walls of 

 the gonoco3l, and the eggs are partly forced into the gonocoel. 

 All of these processes occur as the animals emerge from the sand 

 and swim about preparatory to spawning. 



Spawning extends through the greater part of spring and sum- 

 mer in the warmer waters, and always occurs at a definite time 

 of day about sundown, or from five to seven in the evening. 

 Strong contractions of the general body musculature rupture 

 the cicatrices and force the eggs from the gonocoel into the atrial 

 cavity, and thence they are carried by bodily contractions and 

 the respiratory current out through the atriopore into the free 

 sea water. 



Contact with the sea water brings about the formation of a 

 thick mucilaginous second or inner membrane over the surface 



