166 GENERAL EMBRYOLOGY 



specialized cells, and if they cannot conjugate and develop, 

 they soon perish, not able even to remain alive long, except 

 in a few special instances. Spermatozoa discharged freely 

 into the water, as in external fertilization, are usually able to 

 remain active only a few minutes or hours. But when fertili- 

 zation is internal and the spermatozoa are received into some 

 reproductive cavity of the female, or into some storage cavity, 

 they may remain alive and able to function under appropriate 

 conditions for a much longer period: various observers give 

 the following specific instances: dog and rabbit, eight days; 

 man, seven to twenty days; fowl, twenty days; the bats and 

 some snakes, from autumn to the following spring; Salamandra 

 maculosa, from summer to the following spring; snails, three 

 years; bees, four to five years. 



Whatever the particular circumstances connected with and 

 ensuring the meeting of sperm and ovum, the medium in which 

 it occurs is a fluid. In this the sperm cells are in active, though 

 apparently random, movement, due to rapid vibration of the 

 flagellum or tail. In many instances their movement follows a 

 spiral path (Buller, Adolphi), either close or open, such as is 

 common among flagellated unicellular organisms. In a few 

 rare instances (some Crustacea) the spermatozoa are amoeboid. 



Fertilization becomes more likely when direction is given to 

 the active random movements of the sperm cells. In some 

 instances where fertilization is internal, the movements of the 

 sperm seem to be directed by ciliary currents of the oviduct or 

 other passage. Spermatozoa tend to swim against such a 

 current, and thus to ascend toward the eggs which are being 

 carried down the passage toward the exterior (Lott). Accord- 

 ing to the interesting observations of Lott, human spermato- 

 zoa swim at the rate of 27 mm. (i.e., about 550 times their 

 own length) in 7.5 minutes. At the corresponding rate of 

 progression a man 5.8 feet in stature would walk a mile in 

 12.4 minutes. 



There appear to be two chief methods by which spermatozoa 

 are finally brought to the surface of the ovum. In some few 

 forms the egg is said to give off a chemical substance to which 



