THE DEVELOPMENT OF AMPHIOXUS 9 



of the egg. The substance of this, the perivitelline membrane, 

 seems to be derived from the material of the peripheral layers 

 of vacuoles in the cytoplasm; these flow together forming a 

 homogeneous superficial layer quite distinct from the underlying- 

 protoplasm (Fig. 1). The membrane is at first rather fluid 

 but in contact with the water it soon begins to toughen. 

 In this condition the eggs are ready for fertilization. The 

 formation of the spermatozoa is not so fully known but it seems 

 to occur in a manner parallel to the formation of the ova. 



II. THE EMBRYONIC PERIOD 



A. FROM FERTILIZATION TO THE TIME OF HATCHING 



1. Fertilization 



Eggs and sperm are discharged into the water at about the 

 same time and immediately each egg becomes surrounded by a 

 crowd of the spermatozoa. The perivitelline membrane has 

 been formed first in the region of the animal pole and at this 

 time is gradually extending thence toward the vegetal pole. 

 In this region then it remains longest in a more fluid condition 

 and it is at this vegetal pole that the sperm usually gain 

 entrance to the egg, after having penetrated first the thin 

 vitelline membrane and traversed the perivitelline space. No 

 micropyle has been discovered and polyspermy seems to be 

 very frequent. It is not surely known whether the entire 

 sperm or only the head usually enters the egg substance. 

 Immediately after the entrance of a spermatozoon the entire 

 perivitelline membrane is formed, toughens, leaves the surface 

 of the egg, and comes into contact with or even partially fuses 

 with the vitelline membrane (Fig. 1, D) obliterating the original 

 perivitelline space but leaving a wide secondary perivitelline 

 space between itself and the egg. 



Entrance of the sperm affords also the stimulus to the 

 completion of maturation of the egg nucleus, which at this 

 time is still in the mesophase of the second polar division. 

 A\ hile the sperm, lying just inside the egg, is reconstituting a 



