50 PROBLEMS OF FERTILIZATION 



structures and mitochondria of the spermatid; the tail 

 includes an axial filament which arises from the cen- 

 trosome in the middle piece or at the base of the 

 head; this is surrounded by a protoplasmic involucre, 

 except for the end piece, which is usually free. 



IV. THE ENTRANCE OF THE SPERMATOZOON AND THE 

 CORTICAL CHANGES OF THE EGG 



Under the usual conditions of insemination the 

 number of spermatozoa vastly exceeds that of the ova. 

 Depending upon the concentration of the sperm, an 

 ovum may be associated with one to several hundred 

 spermatozoa. As spermatozoa tend, under uniform 

 conditions, to form a homogeneous suspension, i.e., 

 to be equally spaced, their actual aggregation with 

 reference to the ova indicates some form of mutual 

 interaction which has been variously interpreted. An 

 old and favored form of interpretation has been in 

 terms of chemotaxis; the ovum was supposed to secrete 

 some substance that directs the movements of the sper- 

 matozoa toward it. Many ova (e.g., echinoderms, some 

 annelids, etc.) possess a gelatinous external layer which 

 seems to entangle spermatozoa and hold them, and this 

 is undoubtedly a factor in such cases in the aggregation 

 of spermatozoa around the eggs. There is also the 

 possibility in some cases that spermatozoa adhere to 

 ova and thus tend to aggregate, and this adhesion may 

 be specific. Such questions will be discussed later. 



In most species but a single spermatozoon enters 

 the ovum, and as soon as this happens others are in 

 some way barred. Normally such ova are monospermic, 

 but if the ova are injured before insemination, two or 



